Honestly, a Legion's weakness is the player's greed. So many Legions get so focused on the dripping health bars that extreme tunnel vision sets in. So, you can flank with Scorch.
It's not reliable because the set up needs to go off without a hitch, but a Scorch can melt any tunnel visioned Legion before they can react. Put a gas canister on them, or a little behind them so they're still in range of it, light it and watch them melt. If they walk away from you, Thermal launcher, if they walk towards, thermal shield. They can't shield the incendiary trap and it just turns them into slag.
Ideally you'd have to ignite the trap (and hit them with the ignition) before they notice you, but if you do this, Legion will lose every 1v1.
But like I said, this is unreliable. If they see you as you're lobbing gas cannisters, there's just a little, teeny weenie, itsy bitsy chance that you're about to get completely fucking annihilated.