Easiest path is generally agreed upon to be yellow jacket then the vigor path. As for ultron himself, it’d be helpful to see your roster, but a few really good options for ultron include Medusa, Loki (with his sp1 you can steal ultron’s absurdly potent buffs for yourself), and magneto. Believe it or not, ultron can be done with as low as a 2* loki or a 3* magneto.