I used 4 star Void. Wait ultron evades to throw sp1. After 2 petrify debufs is just a normal fight to reverse the healing With good rng on the petrify is very direct to one shot him
Just finished my first ever completion of Act 5. Trialled Ghost first up to see how I went and managed to take off 57% of Ultron, playing really badly. Went in with Fourvus aftwards and roflstomped the rest of it
Loki is great for it. Use the sp1 while Ultron has the power gain buff, then use the sp3 for the curse. After that, just rinse and repeat (the fury buff from Ultron should give your hits enough oomph even at a 5* r1)
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With good rng on the petrify is very direct to one shot him