Placement:
Only use mix towers where they can reach two paths at once, then use the others when they will only be attacking “up.”
Power-ups:
Power-ups are excellent when you group three or four towers around it to take advantage. Make sure they are “mix” towers to get the bonus for both types of enemies.
Upgrades:
Level 3 upgrades are pretty useful and will not get to use them if you are “spamming” towers.