First off... This "Group Boss" that requires more than one agent, does not actually require more than one agent. You can kill him yourself rather easily.
ULTRON WILL RESPAWN 3 DAYS AFTER HE FIRST SHOWED UP.
DO. NOT. KILL. HIM.
DO. NOT. KILL. HIM.
It is in your better interests to instead focus on dealing 10% damage to ALLY ULTRONS.
At level 152 he has roughly 200k~150k HP. That sounds like a lot, and it's roughly 20x~15x an average boss' HP at my level. However, since he's the only one on the field, it's a 3 on 1 and the damage REALLY stacks up. It took me 4 battles to kill him on my own.
BE WARNED: YOU CANNOT USE ITEMS IN THIS BATTLE!!!
The above stipulation (whether your agent is alive or not) makes a battle that you COULD beat in 1 go ridiculously hard (comparatively). You WILL die to Ultron every so often. So if you have heroes that tend to get hit a lot, or require lots of Stamina, your agent will have to handle that through Gadgets. But so long as you deal your 10% you WILL get a lockbox.
My agent is level 152 (at the point I first fought him), and I have a Quantum Jumper, MULTIPLE MFGs, and the Warbringer's Axe. This is all I found necessary for my agent during these battles.
Each time I've fought him, it seemed that the battles lasted somewhere between 4 and 5 rounds. When you see Ultron FIRST gain the "FLEEING" status, that means he will leave TWO ROUNDS after that point.
Cable and Blaster Thor will outrught demolish him. I suggest Avengers Thor over Modern Thor if you have him because doing Strengthened first move is fairly imperitive, and is a wasted move on Modern.
But because of the Blaster Type Bonus you take out enourmous chunks of his HP. When I fought him he only had 19% left, but I got rid of that by second round, with Cable's extra turn, MFG turns, and a QJ.
I have a feeling this team could easily demolish him just like the suggested scrapper team does.
I outright KILLED Ultron in this Mode from >50% with Bruiser Avenger's Thor, and Bruiser Avengers Cap. But bring whatever Bruisers you can that DON'T REQUIRE SET UP.
Don't bother bringing a protector, as his attacks are stealthy. I know I say that and suggest Cap, but this form "counters" melee attacks an almost all Bruisers use melee attacks. I've found Cap and Thor to synergize immensely well in prior battles, so this is what I like to do.
Your team seems to always go first in these battles so, MFG with your Agent (if you have it), Strengthen with Thor, and just beat the hell out of him with Cap (forget Shield Guard). Focus on keeping your heroes alive and buffed with your agent.
2x Enraged (which you WILL get) + Strengthened + Cap's passive + Warbringers + 2 turns a round (via MFG/QJ) you can take him down incredibly easily.
User Suggested Teams
Colossus (P5) and the Hulk (WW). I recommend a Dark Sigil, Coulson's Revenge, MFG and Curative Reach Agent build. - Ikoleidan Reviver
He counters non-stealthy attacks which cause Bleeds on your team.
That's it. Hawkeye (level 9). With full Attack ISOs he can get you the 10% required alone. The man SHREDS Blaster bosses.
Magneto is a popular choive as well, and I can see why. however since my Hawkeye went first and did 9% on his own. It only took one attack from Mags to get him over the threshold to the point where I "Recharged" till the battle was over. I can see these two completely dominating him though. I could probably have outright killed him from the 60% he was at if I was actually trying.
User Suggested Teams
Magneto and Grey Widow. He starts magnetized so gets plenty of pre-counters. - Regulus22
I used Storm to shroud me and Human Torch to crit those fireballs. - DeanRuborn
I brought Tact Grey BW + Hank Pym (2 stealthy attacks). - Shizoida
He's immune to bleeding, so the easiest way to take down Tactician bosses is now off the table (Catty Black Cat + Tigra/Black Panter).
He's also immune to Psychic so Inf Emma Frost and Psylock won't much help either.
I haven't figured an Infiltrator team to successfully counter him yet, So for this Mode I suggest just bringing whatever you can that's not a Blaster.
If you can, bring a hero that can remove status effects, as he will almost always put "Mind Control" on one of your characters, causing them to suicide infront of one of your attacks. (atleast at my power level they get one shot)
User Suggested Teams
Mockingbird and a Pressure Points Hero. - Mckrongs (Deadpool's L2->L6 combo/Captain Britain I assume)
Gambit, Psylocke + Inf Agent with Heavy Ion Beam (sub Stark Catalyst or anything), Coulsons (no sub), Waraxe, Marksman's Spotter. - Potkettleblack
I have yet to encounter this Mode yet. But a team full of Scrappers should suffice. Will update when I find him.
User Suggested Teams
Used Wolvie and either Magik or Ghost Rider - Regulus22
Ghost Rider and Iron Fist. And MFG, Melsalam/Angolob, curative reach Agent. - A Wikia contributor
Farming Ultron for Lockboxes
So here's the real meat and potatoes of Ultron... getting Lockboxes from him.
You can get inbetween 13 and 14 lockboxes from Ultron for each "spawn". 14 is in the event that you manage to hit 10% of his health EXACTLY each time you fight him. However, I would not try for this... I'll explain later.
It would take 10 energy and 20 UISO per battle on your map, but costs nothing on ally maps. Not a bad tradeoff for a fairly easy 1 lockbox.
If you want to farm him then your goal is, in this order, Take him down 10% -> Survive till he flees OR Kill yourself.
On the topic of ALLY Ultrons. You'll only get ONE shot per ally's Ultron per day. So make sure you deal the 10%. Also, think of the Ally Ultrons as "investments". Should you deal atleast 10% to them, you will gain 4 more lockboxes on top of the 1 for 10%. Now if you DO NOT CHECK your allies EARLY, you will instead be greeted by an Ultron with 1% HP that can literally be oneshot. You will NOT be able to gain the 1 Lockbox from 10%, however (it seems) that since you deal the finishing blow (even though he's likely long since dead) you will be included in the 4 Lock Box prize at the end.
This means that Ally Ultrons are actually higher priority than your own. But also, the largest possible farm is at your own. Give up on a possible 14 for 5 instead? I'm telling you, yes give up on the 14. If you get 3 allies before their Ultrons have been killed you'll have netted 15 Lockboxes for THREE battles, as opposed to the 10 on a perfect Ultron farm. Need I emphasize that these THREE are FREE, as opposed to 10 Energy and 20 UISO per battle on your own Ultron?
"But I don't want to leave MY Ultron to the wind for my allies to kill... Wat do?" Simple. Take him down to about 40%, AND LEAVE HIM. At 40% your allies should be able to mop him up fairly easily. Assuming you get 3 allies to help per day (as I have gotten to help 3 allies today) 40 should be plenty. And if your allies do the same, YOU will have a large enough buffer to get your 1 10% lockbox off him while also making certain that he will be defeated for the 4 extra bonus Lockboxes.
If this is pulled off perfectly, you can get 20 Lockboxes in one round of farming. 5 from yours (assuming 1 fight, and your allies carry you to victory), and 15 from your 3 allies that day.
When should I kill my own Ultron then? When the timer is getting close to running out. I would say, no sooner than 6 hours before he runs (unless that would run into when you're sleeping). DEFINITELY not before the last day he's around.
SPREAD THIS KNOWLEDGE AROUND. IT IS MORE BENEFICIAL, IN BOTH SELFISH AND SELFLESS CATEGORIES, TO NOT FULLY FARM YOUR OWN ULTRONS. POST IT ON THE FORUMS AND TELL YOUR ALLIES.
So, steps to farming:
1) Prioritize Ally Ultrons over your own. And go at them as soon as you see them.
2) Aim to STOP HURTING HIM after he's taken the required 10% in damage.
3) For your own Ultron, fight him ONCE (to get him to appear on ally maps) and then leave him be. Take him down as far as you can, but leave him after that.
4) Only kill your own Ultron when he's close to running away (ie. his map timer, not his buff/debuff).
5) Leave your Ultron on your map UNCLAIMED after he has been killed by allies. Only reclaim him when his timer is almost out. Why? Read up on Ultron Etiquette
If everyone follows these rules then everyone will gain more Lockboxes than if they aim to solo him all the time.