Well that was insanely hard. Mech cluster was 6 centipedes (5 with infernos), then about a dozen each of scythers, lancers, and pikemen. They also had five auto mortars, a few inferno turrets, and heavy charge blaster turrets. Both turrets had insane range. A half dozen small turrets rounded out the defenses. I took out the mortars, a poison fog generator, and several of the turrets with HE mortar shells. The scythers were easy to kite into my defenses, but none of the other mechs would. Called in help from an allied faction to draw fire (cannon fodder), tied up the inferno centipedes with melee pawns, then gunned everything else down. At the end, out of 18 pawns only 3 were uninjured, and five were downed. Several missing fingers and toes, one eye shot out, and my only two sets of marine armor were tattered.
Now the quest seems stuck. It hasn't ended and the two cataphracts sent to help are still in my base. All of the mechs are dead, and all of the buildings they spawned ...