In order to target 6 enemies in Adrenaline Rush, you will need to complete 'The More' ability by defeating 4 enemies at once using Adrenaline, on 3 separate occasions.
Once you've unlocked the ability, you'll need to locate 6 enemies together. I found the easiest way to do that is complete a smuggling/gun running contract on Toshara, taking contract item from point A to point B. Those contracts are available on a regular basis, too.
After picking the contract item up, jump on your speeder and then start travelling - groups of enemies will spawn, both on foot and on speeders. The groups on foot will normally spawn in threes and quite often you will spawn two groups close to each other. As they shoot at you, your adrenaline will spike. Jump off your speeder about halfway between those two groups and they should all be within striking distance. I didn't seem to get this trophy whilst riding the speeder, I suggest doing this on foot.
I managed to replicate this almost every smuggling run with little effort - invariably by the time I got off my speeder to activate adrenaline rush I had the two groups of three on foot, plus an additional 2 or 3 on speeders to target. Plenty of fodder to aim at!
You can use the same method for