Buy the second jump upgrade, then hit both his tentacles. Use 2 of your 3 jumps to his his brain. Use the third jump to reach the ladder on the left, then repeat for gem grind.
I was attempting to count the number of MOABs I was facing when I realized that being able to look at a breakdown of the wave (maybe by clicking on "RBE"?) would be very useful.
So the main dude is 50 meters tall on the ground, which is unrealistic by itself.
But factor in the fact that he's 250,000 meters tall in the lava, and you get total confusion.