Gloom is there for the extra crowd control, since the enemies affected by her Silence will take longer to do the animation, meaning she's safe for longer periods. Caralyzing Shields is mandatory, since she's made out of thin paper. A single cast of her 3 will fully max your Shield, which means you'll get max Shield Gate. You can also spam her 2, if you want, since it'll give you extra damage versus enemies effected by it. The idea is to get your Shield back, basically.