since you got the first part right, i see no harm in giving the variable part. Don't kill me cyanide.

state
varset
trigger = 1
var(0) = p2life - life
ignorehitpause=1

state
varset
trigger = var(0)<0
var(0) = 0
ignorehitpause=1

state
attackmulset
trigger = 1
value = 1+(floor(var(0)/10))
ignorehitpause=1

That's obvious what it does, sets the variable to he difference of your life points compared to your opponent's life points. The attackmulset is equal to that difference divided by 10, so if the difference between you lifepoints is 233, you will get a 23 point attack multiplication boost, that could get excessive and I'll leave it up to you to limit it.(if you want to). the floor is there so it can round any number with a decimal back down to the lowest whole number, so if you happen for some weird reason take 5 damage points, you'll no have any increase since 5/10 is .5 and that will go back to zero.

[state -2]
type = targetlifeadd
trigger1 = numtarget
trigger1 = target,time=0
value = ceil(target, life*.7))

it's simple really, 70% = .7. multiply that by the target's life. you must use ceil or floor when dealing with decimals and damage, otherwise you'll get debug spam since damage can only be measured in non-float integers. Same thing with the numtarget trigger you only want this to activate if you hit an opponent and thy didn't parry or something, so you add that trigger otherwise MUGEN will give you debug spam since there is no current target. You might also use the movecontact/movehit/moveguarded triggers but you might not need them since it's a custom state.

[mcode]movecontact
movehit
moveguarded
ceil
floor[/mcode]

first you just make the damage do 250

[mcode]reversaldef

hitoverride[/mcode]

both good for countering.

yeah you need a custom state to set the opponents life it's the easiest way to do it.
[mcode]
lifeset[/mcode]