I have found the most useful combo to be a CPS 2000 and a backpack full of extra water =D !!! Haha. I used that in Vermin wars 5-7.
Alot of times extra guns add more bulk and mess up your agility, although sometimes they don't. Let's see what combos I've used...
Vermin War I: Speed Loader 1500, SS 30, squirt pistol
Vermin War II: CPS 1700, XP 105, SS 30, 50 waterballoons, 5 flower bombs, a squirt pistol, and 2 2 liter bottles to refil my CPS 1700. Believe it or not I used all of this in the war. This was back when we had soakfests though and not 1 hit kills battles.
Vermin War III: CPS 3200 and SS 40. I've used this on other occasions too, it worked pretty well.
Vermin War IV: lots of different guns.
Vermin War V: CPS 2000 extra water
Vermin War VI: CPS 2000 extra water
Vermin War VII: CPS 2000 extra water
Vermin War VIII: CPS 2100 and extra water in a backpack.
Vermin War IX: CPS 1500 and extra water in a backpack.
Vermin War X: CPS 4100, SS 40
Dominator War II: CPS 1000, XP 150, SS 30.