Spark Mandrill's stage can be tough. There's a lot of narrow spaces with enemies in inconvenient locations. For the most part, the things that work well in any% also work well in 100%, but you have some more options in 100% that can make things a bit easier, depending on route.
First hallway Edit
The easiest way through the first section all the way up to thunder slimer is by using tornados on everything in your path. There's not much to say on this method, it's very straightforward, just rush up to every enemy blocking your way and kill them with a tornado.
A faster way through is to use ice shots pretty much throughout. For the first blue guy at the start of the stage, as soon as you land in the stage, swap to ice and fire right before dashing. The shot should hit such that most, if not all, of the shards hit the blue guy. Then fire a second ice shot as you get very close to the blue guy to finish it off. The second blue guy is essentially identical, as soon as you climb the ladder just fire an ice shot before dashing. The third blue guy should just be killed with tornado, there isn't a smooth way to use other weapons on it. The final blue guy ice shot is more tricky, there's a bit of a visual cue for where to fire, when X is crossing over the green pillar during a dash.
Exiting the narrow corridor there's an optional c.sting shot you can do to kill the first flying enemy to help reduce lag. For the jump around the tall platform, release jump right around the peak of your jump. This will get you to land a bit left of the edge of the very bottom platform so you have at least a little space to work with doing a dash jump so you don't have to stop. It's not entirely necessary, you can just as easily stop moving for a handful of frames and continue forward. If you choose to go for ice shots on the flamingos it would be better to practice the no stop method, as it will provide a more consistent setup. If you fire a dash shot into a flamingo, it gives you a 4 frame window to fire the ice shot in a way that will kill the flamingo and allow you to get by smoothly, otherwise the window is small. To set up an ice shot for the third flamingo, fire a dash shot and jump from the edge of the previous platform. This will put you at just the right position to fire the ice shot as you land. The next two are similar, but more tricky since you can't conveniently do a full height jump. With practice you get a feel for it though.
Thunder Slimer Edit
Thunder Slimer has invincibility frames, so he can only be hit every 8 frames. A common strategy used is to jump to his level at the top of the screen and fire tornados until he comes down, then switch to boomerang (or c.sting, but boomerangs are a safer choice) up close since they deal two damage instead of just one.
An alternative, but surprisingly workable strategy that's a bit more forgiving of luck is to just squat in the corner you start in and fire boomerangs every 8-10 frames. Dodge out of the way when he comes down, of course, then resume attacking.
The fastest way to beat Slimer is opening with the HETBOOMNADO, the caps are very important, without them no one will know what you're talking about. How it works is you fire 3 boomerangs (spaced out very slightly, 8-10 frames apart, so none hit him on invincibility frames), jump to the wall, dash wallkick off, fire 1 more boomerang, then swap to tornado and fire. Get underneath Slimer while the tornado is hitting him and check if he's coming down. If so, dodge and go back to boomerang (or go to c.sting). If he does not come down, swap to c.sting to prepare for a HETSTINGNADO, just like the HETBOOMNADO but firing 2 c.stings before swapping to tornado. Repeat this process until Slimer comes down or dies. If you get bubbled at some point, you can just switch to boomerang and fire them for a while until a better opening comes up, there's no need to panic and wiggle around trying to set up your next shot off the wall, unless you see the opportunity is good (e.g. trapped by the bubble with their despawn time a long way away, tornado still on screen, Slimer near the center so he won't block you if you go to the wall).
Second half Edit
After Thunder Slimer, the rest of the stage is simple. That doesn't necessarily mean it's easy, there are a lot of enemies to weave around, but there's nothing fancy at all like ice shots on blue guys or rapid weapon swaps. Immediately outside the miniboss door there's a high platform you need to climb, it makes no difference if you jump off the left or right wall, so right is easier. Swap to tornado (if you aren't already on it) before reaching the next raised platform. To get onto the raised platforms, you only need one dash wallkick. Fire a tornado right before doing the wallkick, then dash jump off the edge of the platform and you should be able to land on the platform the rush roader is on right after the tornado kills it. If the tornado was timed right (and you don't slip up in your movement) then the turret on the ceiling will also get killed. If so, you can dash jump right before hitting the next rush roader and save one dash input getting off the cliff. Otherwise, hop over the enemy, then do a short hop before reaching the edge so you can smoothly fall to the bottom without having to release right at all.
The next hallway has a lot of enemies, but just dodge all of them. If you keep dashing and never stop, the turrets won't have a chance to hit you at all. If this is difficult, you can give yourself a bit of breathing room by killing one of the ceiling turrets with a tornado. At the top of the upcoming climb is another flat section with a big turtle enemy in the way, the best way through is to use the Paul tornado. Fire a tornado as you wallkick to reach the top and, because you fired it while climbing, you won't outrun it and make it despawn. Instead, it will carry through and deal enough damage to kill the turtle, saving you from having to do a very precise jump or from wasting time doing a safer method.
Getting around the next turtle is trickier, there isn't a simple method of just killing it. One method of getting past it is to wallkick the right side of the platform in front of the turtle and leaping over. The other method is to simply try to jump at the edge of the platform and land smoothly on the other side. The first method is more consistent, but the second doesn't carry much risk, unless you severely mistime your jump, all it should cost you is the time it takes you to wallkick the wall behind the turtle and climb up. If you find yourself bonking the second turtle, it might be best to avoid attempting the difficult jump if you have less than 5 health (will kill you if you get bonked).
The final section has a lot of up and down in the hallway, but once you get a handle on how the enemies spawn and where you need to fire, it becomes a lot easier. You can use c.sting the whole time which makes the section easy because it's much more difficult to miss with c.sting's large hitbox, but at the risk of ammo drops. If the weapon swap during movement is difficult it doesn't make a big difference, but avoiding drops that halt the game is better for obvious reasons. The first walker can be killed together with one of the firefly enemies with a well placed c.sting shot. Fire after doing a dash wallkick on the wall before the walker and the bottom piece will hit the walker while the center piece will hit the hotarion, this allows you to just continue dashing through without pauses or bonks. After this c.sting shot is where you would put the weapon swap, once the upper piece gets off screen.
Waterless route Edit
You can start similar to any% with a tornado or ice shots on the first blue guy, then you don't even need to kill the second, just grab the ladder downward to go get the subtank. You can fire a tornado after letting go but before reaching the ground and get the tornado to kill both blue guys in the way.
Another approach to the subtank is to hold a charge after killing the first blue guy, and fire a fully charged pink shot + uncharged buster shots point blank into the first blue guy. Some of the pink shot can carry through to the second blue guy, and you can swap to ice to finish him off or just fire more buster shots. Either way it's a bit faster than using tornado.
Then dash forward, swap to boomerang, jump and fire a boomerang to grab the subtank. When you fire, hold a charge. When the boomerang returns, you have a handful of options for how to get past the next blue guy.
The easy version uses a charged c.sting right before bonking into the blue guy and rushing through as much as possible before the invincibility wears off. When it does, switch to tornado for the rest of the hallway. Hold a charge after killing the final flamingo to prep a charged tornado for thunder slimer.
Alternatively, after getting the subtank, you can use a charged buster shot (plus some uncharged shots), or a charged tornado on the blue guy. Then, proceed in the same way as any%, with the difference being you hold a charge after the final flamingo to get a charged tornado. You can also switch to fire at the end to kill the final flamingo more easily.
The Slimer fight in 100% is very easy, with little opportunity for messups. Dash toward the center and fire a charged tornado, then jump off a wall and fire an uncharged tornado at Slimer and hold the charge. A second charged tornado will kill him.
The next section you can proceed as in any%, or possibly using a charged c.sting. Just make sure you use c.sting early enough that it wears off in time to use a boomerang to grab the heart. To grab the heart, the easiest way is to do a full height jump at the wall below, dash wallkicking off (with no dpad input anymore), and when X is far from the wall, fire a boomerang. It should curve upward, grab the boomerang, and come back down. If the timing is difficult, you can also spam 3 boomerangs out after kicking off the wall, the window for a good bomerang is large enough that 1 of the 3 should be fine. You don't need to wait for the boomerang at all, so if you're confident you got the right boomerang shot just climb down the ladder.
A faster way to get the heart, but more difficult, can be done after using the paul tornado (using uncharged tornado really early to kill the turtle in front of the heart). Dash jump off the edge of the platform the turtle was on, and right before hitting the peak of your jump, release right on the dpad and fire a boomerang. If fired early enough in your jump and at the right position horizontally, it will curve upward and grab the heart without having to kick off the far right wall first.
Optionally, begin holding a charge after firing that boomerang and you can use a charged tornado to kill the second big turtle. You can do a shorter jump over the platform in front of the turtle and smoothly jump onto the second this way.
The rest of the stage you can proceed just like in any%.
Waterful route Edit
Only one major difference between 100% waterful vs waterless routes. The final section can't be done as easily, since you don't have c.sting. You can use either a mid-charge shot or a pink shot to kill the walker and firefly enemy. The mid-charge shot isn't laggy, but is much trickier to do, you need just the right timing to hit the walker, so it will probably require a bit of practice.
Besides that one section, the only other difference is that you have limited options for the first half of the stage, you have no access to c.sting so you're stuck with more difficult strategies.
Mandrill fight Edit
Mandrill's weakness is shotgun ice, and in casual play pretty much trivializes the fight. Things aren't much more difficult in the speedrun fight, but Mandrill can throw a wrench in your plans by doing his punch toward you or jumping if your shot isn't placed properly. Mandrill becomes vulnerable before breaking out of the ice, so you can basically hit him a second time for free. When he frees himself, you want to position your next shot to minimize risk of him dodging, that is, fire at a wall and have one ricocheted piece hit him directly, while at least one goes above him in case he jumps out of the way. If he punches at you, often it will force the ice to hit him too early and he won't take damage, so you also want to be ready to get out of the way.
Optionally, you can open the fight with a full charged (blue if in 100%) buster shot, the only difference being that the final shot will be done with Mandrill trapped under ice, so you can get to the center comfortably before firing (or bonking into him before firing if he's occupying the center).