What settings are you playing with that the nemesis exo is that annoying? High alt-champion-nemesis, I would guess? Or a longer game, if the exos keep getting larger and larger.
Last game the first nemesis exo didn't make it past my minefield, and I won before the second one launched. Diff 9, 60 planet concentric map, and normal champion progress (ie nebulas). Don't remember what level the champ was at when the exo launched, but I never did fully level him that game.