General tips for weapons in particular:
Shotguns are shockingly useful in the right hands, with an almost guaranteed chance to hit at medium ranges and their potential to winkle out enemies hiding in destructable cover or exploit bunched up Melee foes. Barry in particular can easily run up a flank, unload three shots into a cluster of enemies that shred light cover, give them all a 5hp per turn bleed effect and then run back into cover with his free movement perk.
SMGs may be a little weak pound for pound vs assault rifles, but their shoot-and-move ability is about as close to a breach-and-clear move as the game has. Being able to traverse cover and throw down some chip damage, or finish off a wounded grunt you dont want to waste better shots on, is never not useful.
Akimbo pistols are…an interesting build to go for, especially if you’re using your own custom merc and don’t have Akimbo. Nevertheless, stick some compensators and short barrels on some of the early browning pistols and make prodigious use of the move-and-shoot ability handguns have and you’ll easily be able to run up and line up a devastating headshot on most mooks without much trouble. Just have either a rifle or a few grenades on hand to fall back on if you bite off more than you can chew.
Sniping is pretty self-explanatory, but as a tip, if you want a cheap and versatile sniper without paying sniper prices, Grunty’s ability to shoot once at the start of combat can be incredibly useful if he’s on overwatch with a Silenced rifle. More than once I’ve been discovered only to have a sentry’s head burst before he could raise the alarm. What an absolute lad.