When Willo Ults, fly into the air as well to take her down. This makes it a lot harder for her to hit you, especially if you have good aerial movement.
In a 1v1 situation, silence him with running blast and deal damage until he uses emitter, then channel recharge towards him until he runs out of shields then finish him. Nimble would be perfect to keep pressure on him if he tries to run away.
If you pressure Seris into using her escape, turn around and listen for Seris uncloaking. Try getting someone low in the mean time, this will often cause Seris to believe she is safe since you aren't looking at her and will uncloak to heal her team.
Many Bomb King players lay "traps" throughout the maps. When you see sticky bombs on the floor or on walls, shoot them to destroy them! They take no more than 5 bullets to destroy and can save a teammate from walking into it and getting blown up.