Use nothing except mix towers where they can reach two paths at once. Use the others when they will only be attacking "up."
Power-ups
Power-ups are great if you can group three or four towers around it to take advantage. Make sure they are "mix" towers so you can get the bonus for both types of enemies.
Upgrades
Level 3 upgrades are not so useful. You will not get to use them if you are "spamming" towers.
