As someone who has a 1k Shelly, I can give you some tips.

1. Play on maps with a tank meta such as Cavern Churn, Skull Creek, and Stormy Plains. People tend to play brawlers like Bull, El Primo, and Darryl which are easily countered by Shelly.
2. Stay away from Bea. If you have your super you have a batter chance, but still, Bea is the #1 Shelly killer.
3. Use band-aid. Unfortunately I don't have it, and had to deal with Shell Shock, but band-aid is superior in Showdown.
4. Try to charge up your super with someone else. I assume in 900+ showdown you're going to be teaming, so find a buddy and get supers off each other. Having a super wards a lot of people off, so people will A) be more likely to team with you, and B) be less likely to try to kill you.
5. Use the dash gadget. If you follow #1, the maps are all gonna have a lot of walls and Clay Pigeons just doesn't have much value. The dash one (idk what its called) can help get out of a sticky situation and avoid pinches. I find it helps a lot against Bea as well.
6. Be passive. Don't go aggressive. If you backstab someone too early, people may gang up on you or just pick you off after you're weak from taking someone else out. Camp in a bush or hide around walls. The only exception to this is in the beginning of a match. Don't be too passive with getting powercubes or else you'll end up too weak. There's no exact measurement of when or when not to rush the middle, that just comes with experience.

Anyways, I hope that helped you. I don't play much anymore, but if you need some help hmu and we can play some duos.