It's part of how mission ambushes work, they go directly after the responsible player. Stalkers have a similar problem with Hide not working all that great against them.
Try putting your henchmen in Bodyguard mode (Defensive Follow, must be within Supremacy range) to help distribute the damage. You should be able to survive a lot better that way.
Also you can place your henchmen/bots/dead people in the narrowest place just outside the vault in such a way that they make a wall. The hero and buddy will try and get over or through it, totally ignoring the fact that you are around the corner and that they are being melted by your henchmen. Make sure to put them into passive and then to order them to attack.
@Boerewors
First of all, do not move away from the vault area until you have defeated the hero, move to the lobby and there will just be more mobs to fight in my experience, let the hero come to you.
Secondly, block the door as best you can and hide behind your pets, the second you see the hero make sure your pets are in bodyguard mode (defensive follow). No matter how effeciently your pets block the door the hero will eventually get through, bodyguard is key to surviving the initial strikes here.
Also, if you can, do the side mission to break another villain free before you enter the bank, the other villain will really help in taking down both the hero but also the security guards/police/longbow that the hero brings with him/her.
[b]@Nightfly[/b]
One method to survive long enough to kill dangerous opponents is to pop 4 def insps, but MMs get the bodyguard mode which should help a lot.
I use the same method as NightFly and have never had any issues - however are you solo or in a team as Mayhems scale hard on your difficulty and team size?
Altitus doesnt have a cure just more alts
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Also, if you can, do the side mission to break another villain free before you enter the bank, the other villain will really help in taking down both the hero but also the security guards/police/longbow that the hero brings with him/her.
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As well as all the dangerous parking meters, crates and garbage bins that threaten your existence. 
I usually return to the Hallway outside of the vault and throw down Tarpatch after I get the message. Hiding out of Line Of Sight, I order my pets to attack the Hero, the AOE splash damage is usually enough to pick of any minions he brings with him.
Never had any problems with a hero and they've never got anywhere near me.
"Idealism is such a wonderful thing. All you really need is someone rational to put it to proper use." - Kerr Avon
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If you're */Dark slap Darkest Night & Fearsome Stare on him too, Fearsome Stares fear won't affect him but will scare his minions & the accuracy debuff affects them all and its huge apparently, the same as Darkest Night. Even for a MM thats about 10.8% unslotted each (17.28 fully slotted with SOs), so at least a 21.6% accuracy debuff & a damage debuff from DN, which should help out a bit.
I usually Bodyguard with all and when he's coming Tar Patch his route, set the Lich on him & then Darkest Night, Fearsome Stare and get the knights to melee him.
Howling Twilight is another fav for the auto minion stun & the -slow & -recharge against them, nice stacked with Tar Patch.
The guys are right though, let him come to you outside the vault regardless of what secondary you have, have your pets in Bodyguard but ready and have any nasty surprises ready that you can (Traps for example has loads of placables and high level Trick Arrow has 2 too).
A lvl 37 Robot/FF MM? Surly you can take on a hero? Unless ur on a high rep or on a team? Iv never had problems but then again dark has debuffs which help A LOT!
As already said, stay in the vault and block the heroes path was much as u can. Then its a dooozy!
Put Tier 1 on Bodyguard.
Put Tier 2/3 on Kill All.
Stand on the otherside of a door or lift, out of LOS.
Take corpse home with you to stuff.
Drop Tar Patch, order pets to stand in the way on agressiuve and stand out of LoS with fluffy. If he/she does get past your pets being in fluffy's aura normally stops him/her hitting you so much until you have him/her held, feared and debuffed
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They were appropriately appalled
at the discovery of the eight victims
They're now putting it all together.
Women wrapped in silk
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Eight legs, one body, silk,
spider, brilliant!"
For Robotics / FF you should be able to just leave everything on Aggressive and PFF up when he gets close to take the damage. BGMode doesn't work through PFF though, be aware of that.
Then, once the henchies have him tied up move about, drop PFF and use that really annoying repel field to semi-perma knockback him into a wall.
What difficulty are you doing it on? If its not the lowest setting then theres your problem right there, drop the diff should be the first step for any mish type or mob type you find difficult.
Every time I rob a bank, I get killed by the hero (Well nearly every time). They are ignoring my pets and targeting me, even if the bots are blocking a doorway between us.
Is this right or is this a bug?
Stone Pharoah:Tanker Lvl 50 Stone/SS
Robospike:Mastermind Lvl 50 Robotics/FF/Mu
Static Burn:Blaster Lvl 50 Fire/Electric/Munitions
Hidden Burn:Tanker Lvl 50 Fire/Dark
More on the way...