You have to trade it with an Electrizer on it.
graveler doesnt evolve at a level, you evolve it by trading.
Level 18
Level 20.
level 30
level 28
Level 55
Your Aron will evolve at level 32 into Lairon.
it does not evolve enymore when it is a blastoise.
24
Level 31
32
Level 25.