With Brellas, it's best to get really close to your enemies. When you do, you should do relatively well in clearing out enemies. Some specifics though.
Splat Brella - I honestly think of this as a top tier SR weapon. Really good DPS while still having acceptable damage at a decent range. The shield makes for an acceptable Roller substitute during Glowfly rushes. Not quite as good as a Roller, but it still works. You can also fire the shield in choke-points to clear out Chums. Not as good as Tenta, but once again, it works. Splat Brella is just a very effective and versatile weapon.
Tenta Brella - Ah the ever-hated Tenta Brella. How you're so misunderstood. It is a difficult weapon to use. There's three things to keep in mind. One is the usual getting as close as possible. Tenta does pretty good damage at point-blank range. The other is staying mobile. It is really easy to get stuck with Tenta Brella, so it's best if you move and jump alot. Kinda like a Blaster. Finally...fire your brella! Tenta Brella's shield is excellent at crowd control. Stick it in a chokepoint, or a place where enemies spawn, and watch them all die as you're able to take care of those pesky bosses. Also Tenta Brella's shield works wonders in a Cohock Charge. Fire your shield and watch the Cohock die (or atleast take alot of damage)!
Undercover Brella - Another misunderstood weapon. I would not recommend attacking with this anywhere farther than point-blank range. Otherwise it does little to no damage. When you are at point-blank range, you have a really good crowd control weapon. It mows through Chums and Smallfry. It also does a decent job at killing bosses. The shield doesn't really do much. It's still alright for Glowfly rushes and Griller waves though. If it is a Griller wave, and you have the Undercover Brella, take out Smallfry. It's your calling!
How long has it been since they gave us Dynamo? We have been given every other weapon type for the past few loadouts, but it has been a long time since I've tasted sweet victory with the Dynamo. I miss it.
Next loadout looks godly. Splat Dualies, Trislosher, Splat Roller, and Squiffer. Too bad it isn't a weekend loadout so I can do it all day.