Grimer in Arena let the game crash

For a few users, cleaning data and cache seems to solve the problem, not for me, though... (this is the source for this).

There is also the small probability that submitting an official bug report might help, you can do so here.

Update: updating to version 0.37.0 seems to fix the Grimer bug for most devices.

This seem to be pretty frequent bug affecting many mobile phone models. It crases the Pokemon Go program whenever the Grimer's animation gets cached. This happens if you click on Grimer in the pokemon storage or in pokedex or if you click on adjacent pokemon in the storage or pokedex. Also it seems to happen if you go to gym where is Grimer or if you attempt to catch it. One of known workarounds that caches the animation correctly (and completely cure the bug until it is un-cached) is that you go to pokedex, go to ca 15th (existing) pokemon before Grimer, and then slide 15 times to right to Grimer. So you can theoretically do this before you approach the Gym and then you can beat the Gym normally. However you need to have Grimer in pokedex, so you must have it hatched.