First of all, there is no point in using something that is not fun to play, and with the usual reload time its unbeareable for me. Thats the point of Tactical Pump and Shotgun Vendetta, it makes much more fun.
Savy with corrosive/heat/slash from [Hunter Munitions] works on all factions, stripping armor and directly damaging health.
Just remember to always hit someone close (well, its the point of being a shotgun anyway) to keep vendetta active and watch them melt!