This line looks like it's trying to define a variable name as "GetTargetActor()", which won't work.
ActiveMagicEffect GetTargetActor()
Your script should extend http://www.creationkit.com/ActiveMagicEffect_Script, I believe, which would allow you to associate it with your magic effect. Try this:
ScriptName aaBess extends ActiveMagicEffectEvent OnEffectStart(Actor akTarget, Actor akCaster) akTarget.PlayAnimation("ihm_attackpowerright")EndEventDoes that code compile? If not, what error does it give? If it does, does it work when you associate that script with your magic effect?
Cipscis