Cut Out
Cut Out, when cast, removes the forest from the target tile, turning it into a non-forested tile. While somewhat useful on its own, Cut Out really shines when used in tandem with Orcish skills, which cause it to give you a bonus to your gold every time you use it.
This is absolutely vital, and the first skill you need to take. With Cut Out, you will have a much larger pool of gold available for tower construction.
Meteor
Meteor is a direct-damage spell, dealing plenty of damage to every enemy in its area-of-effect.
You should never, ever, cast Meteor.
It’s not that Meteor isn’t powerful and useful. It can wipe out several enemies at once, and save you in an emergency. But it will deny your towers the experience they would gain from killing the enemies themselves, and that will cripple your ability to build advanced towers.
The only circumstances under which it is a good idea to cast Meteor are emergencies – when your defences have failed and the enemy is, against all odds, in the cave in large numbers, eying your precious gems. Even so, you’re better off avoiding these circumstances altogether.
Frenzy
Frenzy is a buff spell that, when cast, gives all your towers a huge bonus to their firerate for five seconds. Frenzy is a good emergency spell like Meteor – however, because it does not prevent your towers from gaining experience, it’s far more useful and versatile. Frenzy should be used whenever you are facing a particularly difficult wave – champions in particular.
Don’t use Frenzy at the start of a ninja wave, by the way. It’s a shame to see the ninjas become targetable again just as the effect ends.