Chose Earth one and after that got Water and Air respectively, need only Fire now.
Earth tamer can cause some headaches but he is easy with good tactics. Here's mine:
Against elemental I use Gold Beetle, but any beetle is the same. Beetle is faster than him so start with Swarm, he uses Stun, critters can't be stunned but it will interrupt Swarm and leave him exposed with 100% increased dmg taken. Just take him down there and use Swarm again on Sludgy. Beetle should die during swarm probably and leave Sludgy with 100% increased dmg taken. Enter your beast pet, I use Darkmoon Monkey or Kun-Lai Yeti but any should be fine and finish him easily. Do some damage on the rat and finish him with your last pet.