use cooldowns when you know that you are about to receive heavy damage. For example, you would use Featherfoot when you know a mountain buster is next in Titan's rotation or if you were about to be attacked by a lot of enemies with physical attacks.

you shouldn't have to cycle, but you can usually tell when you need to switch targets based on the damage targets are taking and you know you don't have a good handle on one taking a bit of a dip.