Start by picking you best monster and getting started. No matter who you pick, either Geon or Woo will be the first monster you fight
Geon
Geon simply walks around. I'll give you tips on beating him. Geon is best beaten with Rocky because Rocky is Geon's weakness. Geon moves relatively slow so that you can have a very slight advantage. Beetlemania is also good for fighting Geon because he has an easier time slipping out of Geon's moves.
Woo
Woo is a gigantic gorilla that is fast and slightly stronger than Geon. Woo's weakness is Astroguy, but Rocky is also a good character to fight with. Woo's taunt is longest, so you might be able to use this to your advantage.
Poisonghost
Poisonghost doesn't have a weakness, but Geon can slip out on his moves more easily.
Rocky
Rocky's weakness is two monsters actually, Woo and Beetlemania. These monsters are very strong against Rocky, but rocky is especially hard to finish off.
Beetlemania
There is not much to say about Beetlemania besides the fact that he is weak against Woo
Astroguy
Astroguy is a remarkably fast monster that is weak against a little bit of everyone actually. His taunt is the shortest so that he can hit you more. I must advise not to be hit.