okay, this is my opinon based on what i see in pvp and pvm. I'm not sure about 1hkoing as most bosses are physical resist. But with enough AP and air, gale and tempest could potential outdamage tetra by a lot.
Even without gale/tempest, I'd still go for a gladiator, not a mercenary
They can both boss hunt easily since on a gladiator Chi Sword + Bezerk is crazy. Second of all, gladiators have tons of AoE's so you can switch to air if you like, or turn to water/earth/fire etc. Makes it much easier to train. It is much easier to kill soki with a gladator since it's physical resist.
If you can't get enough AP/attr for the skills to be stronger than tetra/uppercut, you should still stick with gladiator because gladiators are much easier to train than mercenaries who have to depend on getting Plevel'd or grind w/o AoE's in the higher levels; In other words, it is not worth it to go merc just to kill a boss a few seconds faster.
With enough AP/Attr, you can easily kill any boss.