...Which I own, but I don't know where it is or what it's called.
It was probably early 90's, maybe '91 or '92. Isometric view. You controlled a mech with various weapons that had limited ammo, and the goal of each level was to roam around and destroy all of the enemies, and then fight a boss. You were timed. The first level was a city, I think the second was an elevator, and one of them was a train with tons and tons of turrets. Another level was set in ruins, and the boss was 3 (I think?) smaller mechs that created a ring around you and flew really fast.
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Final Zone?
Not everything fits, but it's the only isometric mech shooter on the Genesis that I can remember.