No More Playing With Fire Achievement Guide
For this you'll want to be aiming at the fuel canister on Liam's back, the way I did it was using the Explosive Bolts.
However, another good tactic is to use the Smoke Bolt, which will stun Liam for a certain amount of time, depending which Smoke Bolt upgrade you have. I tried this after I unlocked this achievement and it seems easier with the Smoke Bolts.
Hope this was helpful.