Suffering.

While the giant slime can't move, it can create tentacles that it uses to attack anything that comes close to it. All in all, it won't be an easy enemy to defeat.

Even Kelvin couldn't defeat it reasonably. If he used Life Steal, he would be able to drain its life force and kill it. But killing it that way will make it obvious that he has another ability that is not related to shapeshifting.

He could only use his Solid Material Shifting ability while the slime was immune to most physical damage. So he was helpless against the slime.

The best that he was able to do was pass through its body and escape. The acid burned his skin, but it couldn't kill him.

He was good as new after healing himself. But he wasn't happy with the way things turned out.

He said to himself, "It looks like I will suffer in this part of the obstacle course."