Yeah, Gunner overclock + pump shotgun turns it into a machine gun that melts boss aliens even on insane, just have the whole team using that. The meta for Insane is Gunner cus damage, Tech cus the coils slow aliens down + flame turret damage then Recon cus the drone gives ammo which you lose a lot due to Overclock + the pups thats spot enemies which slows them and do more damage while the wallhack is active.
When I started I went for Demolisher cus he looked like the class to go for damage but he is not, Gunner is just better, those heavy explosive weapons can do good vs waves of aliens but they are terrible on higher difficulties due to the friendly fire damage. And the heavy weapons run out of ammo fast, need a Recon in your squad to maximize them. With the ammo hoarder card he might work better though.
Doc can help up to Extreme but above that damage is more important, you don't need a healer if your are melting aliens with crazy damage before they can touch you. In lower difficulties the Doc can become an overpowered tank with that perk that gives him extra 150 hp every time he gets hit after 10 seconds. You take 20 damage per example, you get 150 in return, you can keep doing that until his hp extra bar goes up to 5000 hp. On Insane the damage from enemies is just too high to abuse that perk.
Phalanx... I didnt even bother trying it on high difficulties, I didnt feel like the perks and builds I made would be effective, the other ones are just better. Damage, slowing aliens, ammoa and spotting are just more usefull than anything he has to offer.