Me, after the task 6 defeated Ultron Group Bosses lets just say (UGB) and noticed that to reduce his health% not only Damage is needed but Number of Hits plays an important roll too.
So i'm making possible team-ups to reduce his health atleast by 25% per round
I defeated him in 4 runs and under 15 minutes
Black Widow(Avengers Uniform, targeted)
Spider-man(better if black costume- exploit webs)
Deadpool (Bang bang bang! - 25hits)
Storm (best, since a DOT and Static charge for extra damage)
Havok (stamina is not a problem, almost guarenteed hit)
Valkyrie (accuracy is a problem, but health is not)
Human torch(bruiser - since his attacks are ranged and if ultron has burning, HT's fireball will reduce 4~5% health at once)
(didn't showed up)
Hawkeye (Useful in both alternates, rapid fire makes worth many hits and damage)
Black panther (ulron is immune to bleed but BP is only tactician who makes many hits)
According to me, Punisher(i don't have him) is best for every ultron)
Utron flees around 3rd~4th round
Agent must carry tyrant blade, and Quantum jumper(its good if u have one) and any multi hit damage maker. If u got Warbringer axe it is good too.
Many people will say Wolverine, Scarlet Witch, Captain Britan are also good, but Ultron is immune to bleed, SW takes many time for Probability field and stamina is the problem, Captain Britain (i can't say i have him at LVL1)
But no offence, its ur team & ur choice!