Changing arrays

Can you chage arrays that already exsist? Like I’m trying to get 1 guy to randomly attack 1 of 5 guys that are grouped together. I’m not sure if this code will work but I’m gonna try it anyway:

 
var pick_enemy_5battle = new Array(0,1,2,3,4)

So that will make an array and using the type of thing Xin (if anyone remembers it) does:

 
var attack_select = new Array(2,42,106,152);
gotoAndPlay(attack_select[random(4)]);

I would do something like
_global.enemyhp+pick_enemy5battle-=attack
So I have 2 questions,
1.Would this code work correctly?

 
var pick_enemy_5battle = new Array(0,1,2,3,4)
_global.enemyhp+pick_enemy5battle-=attack

2.How could I have something like (in the enemy mc)

 
if(_global.enemy0hp<=1){this.removeMovieClip}

or something similar
3.Would removing a movie clip inside a movieclip affect future movieclips or just that one?
4.If an enemy died I wouldn’t want the guy attack a dead person, so how could I change the array to something like 0,1,3,4 instead of 0,1,2,3,4
If anythings unclear just tell me and I’ll try to explain better. Thanks a lot =D