Problem with Experiments

I've been told numerous times that experiments are one of the best sources of fast exp in the game. Recently i completed the Dr.Fenkenstein quest to acquire access to his experiment cave, and i was all prepared to start botting in the cave to get some fast str exp.. the problem is that for some reason i cannot attack the experiment i initially killed for a key during the quest... and its a 1/3 chance i always attack this particular one.

As soon as i turn my bot on, it kills one, and then attacks the one i apparently cannot kill.... so this training method has been deemed completely useless in my own eyes. Am i doing something wrong? Is there something i need to do to work around this problem? Am i training in the wrong cave?

If anything- please recommend me a better npc than Hill Giants- which give 31k per hour, have decent drops- and do very little damage.

From runescapewiki;