This answer assumes you're playing with gen 9 mechanics, you're not allowed to change your Pokemon's moves, and you still need memories to change Silvally's type. If these assumptions are wrong, please tell me and I'll either write a different answer or put this question back on the unanswered list.
In AAA, the viable Pokemon with the most similar stats to Silvally are Mew, wash Rotom, Alolan Muk, and Corviknight, and all 4 of these Pokemon usually use defensive movesets with either a 50% healing move or regenerator. Silvally can't learn any 50% healing moves, so I recommend this moveset.
Silvally @ steel memory
Ability: regenerator
Tera type: ghost
EVs: 252 HP / 252 Def / 4 SpD
Careful nature
- toxic
- parting shot/U-turn
- defog
- multi-attack
Toxic can threaten both offensive and defensive Pokemon, parting shot and U-turn let Silvally sometimes switch after the opponent moves and take full advantage of regenerator, and defog is useful on a Pokemon that resists stealth rock.
source
source