I'm bored, so I'm going to do another run of this: Medium 100%.
Enemies and projectiles in medium are slower (which isn't always helpful, since slower projectiles can block passageways for longer periods), but more importantly, both players start with 3 characters to select and need to rescue other characters along the way. The goal of a 100% is would be to rescue all of them.
Rescue list is:
1-1: Glow Knight, left
1-2: Omni Archer, mid
1-5: Omni Boomer, mid
1-6: Fire Knight, left
North Laser, right
1-8: Side Rifle, left
2-1: Omni Missile, left
2-3: Wind Knight, left
2-5: Water Knight, left
North Boomer, right
2-6: North Laser, right
North Archer, right (blocked by North Laser)
North Spear, right (blocked by North Archer)
2-7: Flamethrower, left
2-8: North MG, slight left
2-9: Spread missile, right*
3-1: Omni Archer, left
3-2: North Archer, left
3-4: Spread Missile, mid
3-6: Side MG, right*
3-7: North Boomer, left
3-9: Omni Grenade, left
4-2: Wind Knight, right
4-3: Omni Missile, right
4-4: Water Knight, mid-left
Fire Knight, left
Glow Knight, right
4-6: Omni Boomer, left
4-9: Side Rifle, left
Spread Missile, right
(*sides are which side has more access, not where they are on the map)
The biggest route-planning constraint is who gets Verbal (omni archer) in 1-2, since there are no other archers until 2-6, and the second Verbal isn't until 3-1.
The main argument for P1 are that 1-8, 2-1, and 2-6 are bottlenecked on P1 rescues, which means that the less time P1 has to spend clearing, the better. P2 is probably a better choice though, because P1 can get the 2-6 Mars (north archer) much more easily than P2, 2-9 is extremely dependent on P2's clearing ability, and the Verbal rescue on 3-1 is on the far left side.
The other constraints are kill-offs to reduce selection times. This is complicated by the fact that bottlenecks now lean more on individual players due to rescues, a player with a rescue needs to spend as little time clearing as possible and also will delay the stage if stuck selecting. Players with rescues also can't suicide.
P1 Athra: P1 has no other runners until 2-7, so Athra can't be killed until then, and even then, there's a tradeoff in that Mars can only shoot his special north. 2-7 and 2-8 are both P1 rescue stages, so the only viable suicide stages on 2-9 and 2-10. P1 is mostly useless on 2-9, so that's probably the best place to kill P1 Athra.
P2 Athra: No-brainer, kill on 1-3 for free.
P2 Gascon: This is a hard one. Pretty much every stage from 2-8 onward is very range-dependent and 2-9 in particular is heavily bottlenecked on P2's clear speed. It costs ~26 frames per stage to keep Gascon alive. Because 2-7 is bottlenecked on the mage kills and the mages have a windup, killing him there would probably cost over 150 frames. Finding a stage where his combat ability isn't crippling is difficult though, and P2 has to rescue on 3-4 so he can't suicide there. The best option looks like biting the bullet and killing him in 2-7.
P1 selecting Robby in 2-9 is also still viable, the only characters P1 has before him in selection are the north grenadier (who is in the default roster) and potentially the omni boomer on 1-5, but that character is a wildcard that can be rescued by either character. Unfortunately, suiciding on 2-5 isn't viable this time because 2-5 has 2 rescues.
The best option appears to be leaving the north grenadier alone and just having P2 rescue the boomerang on 1-5.
The final contention point is that 5-8 might be faster with a knight because knight basic attacks do 2HP damage. This should be easy to access: P2 Mars can probably be killed easily in 4-4 due to P1 needing 2 rescues, and P1 won't have Mars, so picking a knight only requires scrolling past Verbal.
So the planned critical points are:
P2 rescues Verbal in 1-2
P2 kills Athra in 1-3
P2 rescues the boomer in 1-5
P2 kills Gascon in 2-7
P1 kills Athra in 2-9
P2 kills Mars on 4-4
