War Mech is a super powerful random encounter, not an optional boss, since you can theoretically fight him as much as you like. He's pretty much like Iron Giant in FFII, Flan Princess in FFIV, and the Brachiosaur in VI. Omega is something you see on the screen and are given the choice to deal with it (and usually die from it) as well as Shinryuu who guards the almighty Ragnarok Sword. Yet their both one time deals and you even get a silly key item for beating Omega that just congratulates you for winning. Warmech just dies and you go on with your life unless you accidentally encounter another one.