Wesker would have to get through their shields. Wesker also can't hit Lilith if she Phasewalks. If he targets Lilith first and manages to kill her then he's caught in Maya's Phaselock and can't move and becomes an easy target for the rest to hit. Axton and Roland have turrets with homing, Mordecai has Bloodwing and is an insanely precise sniper. Brick has his fists, Gaige has Deathtrap.
Krieg is able to set himself on fire and Brick punches people into mist. Lilith can Phasewalk/teleport. Lilith also can't be touched once she phasewalks, nor seen.
How does Wesker beat Lilith? How does he kill her?
Axton and Roland have turrets that have shields on them to block incoming fire. Wesker had trouble with two people considerably weaker than the vault hunters. How does he beat this whole team when he couldn't take Jill and Chris or Chris and Sheva?
I guess he can kill the others a leave Lilith to the end, if she phasewalks then that should be self-bfr since if she becomes tangible again Wesker would kill her easily.
He didn't have problems with Chris and Sheva, he was toying with them until they started to use the darkness in the environment and rocket launchers (which only stunned him for a bit).
In regards to Wesker vs Chris and Jill, he stomped them in lost in nightmares, Jill had to surprise him and almost sacrificed herself in order to save Chris.
There are really no canon feats for the shields outside of they protect you from bullets and damage. They can be worn down, they regenerate.
Lilith Phasewalking isn't self-BFR, if that's the case then Nightcrawler would self-BFR every time he teleports. When she Phasewalks, she's still there, but not there at the same time. In BL PS she is phasewalking the entire time and tracking down Jack's Vault Hunters. The only people that can sense her are the aliens themselves. She's invisible. Even if it's down to just those two, all she has to do is Phasewalk and pop out to kill him whenever she wants. He is pretty much a sitting duck.
They have RPGs it was one of Brick's specialties. Why can't they just replicate that? Wesker got surprised by Jill, I think he's going to get surprised by a Phaselock or homing turrets or a huge killing deathtrap, or even a murderous bird.