To unlock the Can't Touch This trophy you need to first unlock the Precision Evade ability and then use it 100 times.
After unlocking Double Jump on Kashhyyk, you will be able to spend a couple of Skill Points on this ability. You need to tap dodge (O) right at the very last moment before an enemy's attack lands to get a Precision Evade. You will hear a small whooshing noise when it's done correctly – you'll also be able to tell by the fact that you weren't bashed in the head by the enemy.
Later it becomes much easier to tell if you have successfully Precision Evaded, because you can unlock a Skill that causes a Force Slow effect on the enemy if you have successfully evaded him. The window of opportunity to evade is higher on lower difficulties; but if you are on a harder mode precision evading becomes a pretty critical ally anyway. Chances are you will unlock this during natural progression or soon after. If not, head to the baton stormtroopers dotted around the first portion of Zeffo (the Abandoned Village) for an easy grind.