I was trying to make a few boss battles with events but I'm stuck on a few. I wanted to event reactions as soon as the boss was hit but I don't see anyway to event this. Is this possible or will I have to script it?

Examples.

When hit counter attack target.
When hit change state inflicted.

Nope, you're gonna need a script, most likely. It seems like there's something in the back of my head that makes it so you can, but I'm pretty sure you'll need a script.

The only way I can think to do it would be ridiculously complicated and drawn out....

Basically, just before that boss fight, replace ALL the characters relevant skills with exact duplicate skills, except these skills have a common event call in them that turns a switch on.... then off. So Event is switch 10 on, wait Switch 10 off.

Then whenever you use the ability's you want to trigger the reaction they turn a switch on, ie counter target = switch 1, all states = switch 2, Cross cut = switch 3.

Then in troop page have his reaction on pages that correspond to that switch.
It's pretty simplistic in its outcomes but I jsut tested it and it works.

You could probably make a much more involved system, where the switches control a variable that set the switches the enemy reacts to.
...

I'm probably just rambling.

Well what about just normal attacks?

Yea I couldn't figure out a good way to do it with events so I went into the scripts and made some edits. Almost got to do everything I wanted done except one specific boss. But I posted in the Script Help section for that. Just wish the conditional branch had more options to it.
Although that gives me an idea if its possible. Can you use the conditional branch script section to constantly check to see if the boss's hp has dropped? Like say something like this...

Battle Start
Enemy.hp = Variable1

Conditional Branch
if Enemy.hp < Variable1
Variable1 = Enemy.hp
Cast Spell - Execute now
end

Im pretty sure that could work, however Im not sure how to reference the boss or make sure it is checked constantly instead of at the start or end of a turn.

Indeed you could! But that's probably another question for the RGSS Script Support section.