The Beam Trick Edit
Short, but powerful enough if you've never heard it before that I thought it deserved its own section. You can start using this as soon as you unlock the Beam skill at D4; it's even better when you have the relevant gem skill to boost that special even more.
Gem (in tower) + Beam + Targeting = Super powerful special until Beam wears off, because the special gets re-applied approximately every frame.
- Orange: Random target; huge mana gains
- Green: Random target (or Flying Ones for shadow); huge poison damage - great for taking out shadows, super-tough giants, or anything with a ton of armor (since poison ignores armor)
- Purple: Random target, or Highest Armor target (or Flying Ones for shadow); huge armor shred, great for shadows (which spawn with lots of armor, and gain it each second they're alive)
Beam also raises the hit counter on a gem very quickly, so it's great for improving black (bloodbound - gain power as they have more hits) gems quickly. (It's also good for getting the achievements related to having a low-grade gem with a lot of hits.)