Collateral Damage
Had a Lost Legion Eternal kill 3 of his friends.

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At the beginning of Tycho's Ribs is probably the best place to get this trophy. There will be a lost legion Eternal that will spawn from the building on the right. Damage him a bit and hope he turns to a Magus Eternal or a Unstable Eternal. If he doesn't then save, quit and reload and you will be back at same spot. Just have the Magus Eternal hit the enemies with the slag orbs he shoots at you and he will kill the enemies. If he turns Unstable then just try to run towards him bringing other enemies with you but be careful that you don't get too close else the blast will kill you to. You also may need to soften up the enemies, especially if they have shields and it's also advisable to attempt this trophy on Playthrough 1 as this area can be very tough in TVHM. Here's a video tutorial as well (credit to PSTHC for the video):