I think you're going about this the wrong way - if you want a visual effect on a per-monster basis it might be easier to have obj_battlemonster handle it? It's already aware of which AMP data block it owns so it's a bit easier than going through the control (who manages a ton of monsters at once) Plus, with your current approach you'll create a new shield effect object every turn - if you don't remove them afterwards you'll end up with dozens or hundreds of them in every battle...
In the draw event, add some new code like this:
if(state == bmonsterstate_NORMAL){ //Monster is guaranteed to be alive and valid in this state
if(global.active_monster_party[amp_id,amp_SHIELDHP] > 0){
draw_sprite_ext(spr_shield,0,x+dx,y+dy,image_xscale*drawscale,image_yscale,image_angle,image_blend,image_alpha)
}
}
There's ways to refine it, like adding effects whenever the shield status changes (broken / regenerated) by keeping track of the status the previous step and doing stuff when it's different from the current step. But the important part for now is making it draw properly.