This achievement his hard to get early on, as bosses have high health and your units simply don't have the damage output required to kill them quickly. Additionally, many bosses have reinforcements and scripted run-away events that will slow the fights no matter what you do. Also, it seems that either all enemies with the big red health bar do not count, or the fight duration is timed incorrectly, as sometimes you can kill a dozen bosses in 15 seconds without getting the achievement.
Generally, it seems that vehicle bosses are fastest to kill, at least once you have Terminator armor for Thaddeus (or even Tarkus), a good missile launcher for Avitus and a power fist or thunder hammer for the Force Commander. Terminators (immune to knockdown) standing around the vehicle will block it, and the numerous power weapon hits should keep it stunned anyway. If Tarkus is not in Terminator gear, he should have a plasma gun and melta bombs. Open this with an Orbital Bombardment (use the Force Commander, protected by Iron Halo, as bait to keep the boss still) and you should be good.