There are two ways to deal with STELLA; stack defenses with Mash, Boudica, Waver, and the like (bring CEs like Holy Shroud of Magdalene, Melty Sweetheart, or Iron-Willed Training), then Arts spam to get up to 100% Def up before a STELLA goes off (debuffing his Attack works in combination, too!); or bring along a ton of Guts CEs and Servants like Irisviel, Leonidas, Blackbeard, or Bond CE Heracles, and have a Guts for the team ready to go by the time STELLA hits.
Worst comes to worse, if you have a MLB Holy Shroud of Magdalene, stick it on Mash, pair her with trash, and make damn sure you can pull off Lord Camelot often (use the taunt!) for the solo.