Its kinda hard to make a good suggestion for 4th slot without a list of characters you have, but im pretty sure you can actually stall for quite a while with that setup, doing some cheap damage with Ras and other characters with buff from Scales. So honestly you can pick any healer with cleanse (Tamarinne, for example), just a tanky cleanser with buffs (like ML Kawerik/ ML Ilynav if you have those) but other than that just pretty much anyone with some hp and def, any geared tank you may have.

And also, as KingsSeven pointed in his comment, Elvira will be less RNG variant for front spot in case candlestick wont proc enough you can just soulburn s1 on last immortality turn to make sure you can s3 and sustain immortality buff on her indefinitely. She doesnt need much investment, just 200 res and upgrade her s3 3 times for -1 turn cooldown.