Best for PFF MMs are /em blasters who with aim/build up and powerboost can cut through a PFF like nobody else.
However I simply kill all of their pets now as PFF has a 2 minute downtime it means they ahve to go off and hide while they resummon (and of course you follow them and wait)
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Dev blasters can make a pretty good job of forcing there way through a PFF too. Also, debuffs like snow storm seem to get through it regardless. Could find someone to take your place in the duel and shut the cocky MM up.
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Haha, in the arena?
He doesnt have a hope vs your stalker. Hopefully you have stealth so stacked with hide he will need to have yellows to see you.
As soon as the fight starts charge over eat X amount of yellows, (should test on a friend how many you need vs 3 slotted PFF), buildup and AS him.
Chances are he will not have had time to summon many pets and from then on he will not get much of a chance.
After a few kills if he ever does get his pets out safe way is just not to fight, let the time tick away, and bask in the glory of your early kills 
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Least thats what I would try to do.
Outside of arena just run back and forth from a shop with yellows, reds and lucks and one shoot him each time 
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Edit: Actually on second thoughts he will prob have force bubble wont he 
. Heard that MoG can be used against that but dont know for sure.
As a Bots/FF MM myself in Warburg using PFF is a bit inefficient and I much prefer Force Bubble - only switching to PFF when some serious ranged damage is incoming. Use Unstoppable - it'll nullify the Force Bubble (gives you Repel resist) and allow you to waltz right up.
Best way to defeat me? Drop my toggles - I had a good run the other day until I got duoed by a Dominator stacking holds on me, followed by a Stalker with an AS. Insta-drop.
If I'm just turtling in PFF then high accuracy with some form of mez (stun or sleep). Careful with FF MM's as we have mez protection with Dispersion Bubble so aim for the mezzes that it doesn't protect us from.
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As for Force bubble being the end all of powers in PvP. Stone takers can walk through force bubble, invul tankers that hit unstoppable will walk through force bubble, any regen scrapper that has popped MoG will walk right through your force bubble, this inclues regen stalkers. Since you can't use Force bubble and PFF if you use force bubble the regen stalker can march through it and AS you. In the early dyas of base raids Force bubble was great cause no one expected it and the small halls of the base made it a total wall people couldn't get around but there are so many ways into it people just realized them and made the bubbler the BIG TARGET early in the raid. Trust me, comming from a level 50 bubbler that used to love it after you [censored] someone off with force bubble once they will find the ways through it and kill you.
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May be of interest but pls dont kill me if its wrong 
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as said, forcebubble and PFF dont stack, if he turns on PFF you can just walk through that forcebubble no matter what AT you are. Judging from your post he only has PFF running. Now MMs always think they are so safe in their precious little PFF shell but it aint that grand, especially with the 2 minutes cooldown.
Remember his weakness is that he cant use any powers on his bots while PFF is on. So if he wants to summon/buff his bots he needs to turn it off. In arena, just be fast and rush him, big chance you get to him before he gets his bots all buffed up.
Now you can either pop alot of yellows and try to hit through it, preferrably with some toggledrop attack or a mez that disp bubble doesnt protect against (fear, confuse, sleep) if you have any. However as zaprobo said, aim for his bots. If you are fast enough, chances are he only has buffed 1 bot and had to crawl into PFF to be protected from you. Best bet is to aim for his assault bot first seeing 99% of the MMs prioritize upgrading him.
After that, you just keep pounding on his bots till he detoggles PFF to resummon em, or he just stays in PFF running around scared with no bots. As a ss/inv brute taking down the bots shouldnt be a prob as bots have no smash dmg resistance.
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In a duel I assume you're going to give him time to have all the bots out and upgraded. If he's turtling don't worry about Unstoppable and just pop a dirty truck-load of accuracy inspirations and wail on.
With PFF up he's going to die in short order as he can't fall back on any other tricks (like Force Bolt/Repulsion Field/Repulsion Bomb for detoggling, Force Bubble for anti-melee, and Detention Field for some breathing space) and as others have said PFF stops him doing anything to affect you except through the bots.
If you get down to about half health and you're struggling to hit still, just maul the bots which'll give you all the time you need in a duel (since he can't run to resummon and has no choice but to drop PFF or just sit there helpless).
Accuracy inspirations
Both your characters have their uses against this MM I think. You claws/regen has the benefit of range, although i'm not sure how long the range actually is. Force Bubble has a 50 yard radius, so you'll have to use attacks that are longer range than that, if claws has those.
Your invul Brute has repel resistance, so you're easily able to run right through the force bubble and hit him.
Which brings us to PFF. I suggest, lots of acc insps, build up and hit him.
One thing to watch out for. Detention field. It's an unresistable semi-hold. You won't be able to attack at all, but can't be attacked either. That's a Bubblers survival tool/second panic button.
Your Rad/Rad Corruptor might be able to take him out. Maybe see if you can test it on someone beforehand to see if it works. Radiation Infection is -Def, and doesn't require an accuracy check, so you can put it on someone in PFF. I've used it to hold MMs in PFF before on my Ill/Rad Controller, although I don't think they'd slotted for def, which is why I say test it first. Slotted with three def debuffs, I suspect it would cause some real problems. If he doesn't have Dispersion Bubble up, you could then try Cosmic Burst to stun him and knock him out of PFF altogether.
Been in warburg past few days with my brute, and although won at 2nd attempt, iv never seen a build frustrate me so much. My brute is ss/invul and i struggled so much againt the mm with the pff on. The mm was a bit full of it, and so much so, guranteed i wouldnt win with any at. We now have a little duel set up for this weekend, and im hoping someone may have a recommended build that will certainly put up a good fight
Iv got a regen/claws stalker as well which i dont think would have much chance due to the leadership pool running. Wat would be the best build for taking down the mm? not gonna run another at off as i have confidence in my brute, but would be interested to know 
Cheers