So what types of cards are best with Element Cannon? Small, cheap Defender cards. Element Cannon turns any defender, no matter how weak, into an easy way to kill huge monsters like Giants or Demons. Take a look at the card Wisp, for example:
By itself Wisp is a moderately powerful card that costs no mana, gives free damage, doesn't waste a card, and is good with Equipment. With Element Cannon out, however, it kills a Giant for 2 mana without even using a card! This is exactly the kind of card you want in your deck with Element Cannon.
Take a look at another card-- Parrot Familiar:
Parrot Familiar's main use is to copy an opponent's Ability card. After that, it's just a useless Parrot that can't even deal damage. But with Element Cannon out you can turn the useless 0/1 parrot into a Giant-killer!
You get the idea - turn any defender that would otherwise be weak into a deadly attacker. The thing to keep in mind is that Element Cannon is fairly useless with larger defenders. Sacrificing a 5/5 defender to Element Cannon is pointless because the 5/5 could have killed any monster anyways.
Although Element Cannon is a "build-around-me" card, it isn't useless without cheap defenders. It can turn a Knight's 3 damage attack into a 5 damage explosion. Generally any defender with 3 or less power at least gets some benefit from Element Cannon being out.
There are some cards out there that are even better combos with Element Cannon - finding cool combos and interactions is what Mage Tower is all about!