Arrows and space, and repeatedly press W (you'll need about 5 keypresses per second to break free with no damage- it's easier than it sounds) if you're caught in R.E.X.'s tail grab. Now, however, you can jump again after jumping, and if you walk off the upper platforms, you can do not one, but two midair jumps. R.E.X.'s attacks vary by difficulty, and his health increases as well- Jump- A standard attack used between every attack, on every difficulty. The chance of sending out a shock wave increases by difficulty, and on hard, a shock wave is guaranteed. On hard he has a variant of the attack where he'll jump straight at you, but no shock wave. If he does and attack from the upper platform, he'll just slide off the platform. Tail grab- R.E.X launches a tail projectile to catch you and trap you. You must mash W to break free before he crushes you. Regardless of if you take damage, you will go flying after the attack. Use this to reach upper platforms. Also, their may be slight music glitches, since once you get caught, the song switches to the lyrical version- I tried to sync them as closely as possible, but nothing is perfect. Orbs- R.E.X. launches energy orbs. 2 over a small area on easy, 3 spread out over the map on normal, and three that are aimed straight at you on hard. Missiles- R.E.X. jumps onto the platform above him and shoots a few missiles at you. On hard, there's a variant where he stays on the ground and shoots a missile that releases shockwaves.
Song is Rules of Nature. Now there are 3 difficulty levels! Dun dun dun! I'm too lazy to actually make City Escape, so I'll bring the boss from it here. Normal difficulty is how he'd function in City Escape.