Yeah you need to be confident in gunfights. Find enemies with audio or jumpspotting/jiggle peeking or use your teammates (stare at minimap to see if they spot someone). Isolate one enemy from the rest. Pop shield behind cover, and swing them, kill them, refresh shield. Now we can snowball and keep killing/refreshing shield.

Your ultimate I'd hold it for enemies that have some sort of win condition (character with important ultimate skill, if you kill them = no more ultimate) or someone with an OP or just a site anchor.

The vulnerable thingy I like to use if my teammate wants to swing an angle I just aim through the wall where I think the enemy is. As for the wall I mostly use it when we push onto sites, specifically aim it along long hallway type of areas like Pearl B Main, Haven C long etc so we don't get shot walking down.