scyl's whole deal is that she's kind of a jack of all trades champion. she brings a little bit of everything to your team, but she's really not great at anything you mentioned.
let's start with the easy one—shields. scyl's shields aren't especially strong because they don't exist. none of her abilities grant a shield buff.
next, lets look at her damage. i'd be quite interested to see some examples of scyl capable of dealing massive damage as you put it, because all the ones i come across are struggling to be any more than adequate. her multipliers are decent, but not great and her base defense is nothing to write home about. even if you built her with high crit rate and crit damage, she still wouldn't have the same damage potential as any true big hitter and you'd be making her a lot weaker overall since she's already fairly multi-stat dependent.
her heal is a nice bonus, but it's nowhere near enough to keep a team alive on its own. it can be a helpful as a secondary or tertiary boost to augment your main healer, but it's certainly not a gamebreaking ability on its own.
her revival is a very useful ability, i will grant you, but revival is just a great ability in general. if you compare her revive ability with others, she finishes middle of the pack at best.
i recognize you're struggling against her any i do sympathise, but scyl is not the balance breaker you make her out to be.