Not an expert by a long shot but let me try to contribute from what I've gathered so far based on personal experience and advice from local pro players (as well as the Atrocitus Discord).
Longer post coming up with character-specific stuff, but let's do some broad strokes first:
We are a bully. Atrocitus isn't rushdown, nor is he a turtle. Don't expect to be dealing a lot of damage even by spending meter -- what Atrocitus excels at is finding his optimal range (more-or-less his sweep range), staying there and basically smacking his opponent into being afraid to push buttons. We work best with our opponent in the corner, and we've no shortage of tools for carrying them there and advancing. As we've discussed on the Atrocitus discord: f223 is to approach, b23 is to whiff punish, d1 (you better convert that into d1 xx db1 on hit) is a panic button.
Against rushdown: Pushblock! Atrocitus' normals are a bit on the slower side (except for d1), so getting rushed down can get pretty scary for us. Your best bet is to spend a bar on pushblock to regain the neutral state, then establish your dominance from there. If you've got Dex-starr out (hopefully you've studied some setups but generally, you want a combo that ends in d2 xx 4) f4 will help a ton in deflecting some of the heat. A sneaky mb b3 or f3 can also help out, but don't do it too much or they'll be reading you from five pages away. Resist the temptation to use the mb air escape(s) because properly using those means you know the exact point in the juggle that won't continue the combo if you spend those two bars.
Against zoning: Arguably Atrocitus' trickiest matchup because we've no projectiles (without Dex-starr) and the bloodnado is a tad too slow. Your default approach should always be the classic crouch-walk (NRS vets will know what I mean), then take advantage of some quality advancing mids (i.e. f223) once you're in range. If they're got projectiles that hit low, just commit to the fact that you'll be taking chip damage and plan to deal way more to them once you're in. Resist the temptation to use the mb roll unless you know the exact timing, because it's got recovery and might just get you punished.
Against zoning, with Dex-starr: Deserves a paragraph of its own! Don't be afraid to press 4 at fullscreen because it's worth it to sacrifice a small bit of your lifebar in exchange for bringing the cat out. f4 is going to be your best friend as you walk forward, don't forget to press it again after it eats a projectile. Against some zoners you can actually try doing bf1 in the air and using Dex-starr's projectiles to cover you -- turning the otherwise unsafe ghetto airdash into an actually viable approach. Or just jump in while Dex-starr makes your opponent play Touhou, but that doesn't look half as funny. (Seriously though, walk forward.)
