When has any slime I'm any game ever hardened? That is not a typical skill slimes have. I think we're trying to train him wrong. He's a controller, he can probably make tendrils or shoot slime or other AOEs. PHM is the defender, it's more likely that he can harden than slime. I say we change slime's training to a series of shiny obstacles and put some delishous looking (this is all relative here) garbage at the end. We will see how slime deals with the obstacles and learn what he can do.
Alternatively, we could put "food" after each, which is probably a better idea anyways. To make sure slime doesn't just eat the obstacles, we could cover parts of them in shiny tin foil.
Obstacles:
Tight spaces: make a small passage between two objects (or use te crack beneath a door) and see how slime gets to the end.
Up and over: Cover the top if a wide obstacle in shiny tin foil, see how he gets the food on the other side.
Hanging trehat: tie an old boot or other rash treat out of slimes reach (use our new rope) and see how slime gets te food.
Keep away: wrap PHM in tin foil and give him some tasty food. His job is to keep it away from slime. See who wins. (The answer is us because they both get training)
Mine field (do this last since you can't reuse this tinfoil): place slime in a minefield of tin foil balls and scraps. See how he escapes.
All of these games should train slime in some way since he has to use problem solving in each one. And if he doesn't do any of them, then he is sadly just a disposal monster, which would be quite sad and really weird.
It's not time to attack Ned yet. Don't worry, it will come, but we have better things to do.