This is a weakspot damage build with the goal of pushing past each phase quickly. It's not a speed build but you should be able to clear phases very quickly. You'll use Mom to focus down weakspots, deal with big boys and use Pipe Bombs to help the bots deal with commons.
SETUP:
Start Act 4 and restart until you get a Barrett or Phoenix sniper. An upgraded Belgian is helpful too. With this build, you won't be able to aim down sights but at the start/shop, be sure to get a 15% weakspot scope on your sniper. Purchase 4 Pipe Bombs or more and drop extras over the edge. (Going down to phases 2 and 3 with 4 Pipe Bombs is helpful.) At least 2 Pain Meds too. Upgrades are optional but I would focus on making sure each gun is stacked and you have plenty of Pipe Bombs and Pain Meds.
HOW TO PLAY:
You can't aim down sights but this is a very accurate build if you crouch. For all phases, you'll want to crouch (to activate Hunker Down), shoot your Belgian, and then swap to your sniper and tag weakspots. Shooting your Belgian and then swapping to your sniper activates Power Swap.
Phase 1:
I use a Pipe Bomb right as I jump down to clear out commons. You can take out the tentacles usually in two cycles. Don't rush, it's not a speed build, so take out big boys and work your way around the map, clearing out commons after each tentacle is taken out. Try to go into phase 2 with 3 to 4 Pipe Bombs.
Phase 2: Throw a Pipe Bomb right when you slide to the bottom. Phase 2 is easy, no real worries, and similar to phase 1, be sure to take out big boys. It's nice to go into Phase 3 with all your bots since they'll (sort of) allow you to focus on weakspots while they clear trash. Just be sure to crouch, fire your secondary (Belgian), trigger Power Swap, fire primary (sniper).