I'm starting to wonder if I should stop putting Sumo inside of HBK-4Gs. Every time he drives one, he dies a slow, drawn out, but ultimately ineffectual, death. (Typically, by the time I'm actually losing units, I've taken out two or more opponents.)
I've found I like Apex in the 4G. Her Gunnery 8 + Piercing Shot (or whatever it's called) = take your 100 damage and like it!
At least, until she air-balls it three times in a row, on 85% to-hit. That left me salty, even though I ultimately won the match.
And also yes, the Flamer ammo was a shocker. I'm experimenting with using Kraken in one, mostly for Sensor Lock, until the time is ripe to rush in and grief the crap out of a target and shut it down where my other three can focus fire (or even just one get a heavy backshot).
In fact, it might be fun to take 2x FS9-A and 2x HBK-4G. Would have to be judicious with your shots, but you could probably do a really mean divide-and-conquer. Especially on The Stacks.