You can't go Wrong with Mercs/Storm or Mercs/Pain.
Storm offers more debuffs can aggro management along with mob management than the other sets with Dark aside. Unfortunately expect your pets to be expendable for the most part, and be prepared for a massive amount of endurance reduction slotting.
Dark offers powerful debuffs from Tar Patch to the extremely handy Fearsome Stare. Supplying the set with plenty of heals and ToHit debuffs that will have entire mobs whiffing away aimlessly. Perfect for a Mercs MM because the pets enjoy staying in range with the MM.
Trick Arrow is unusually fitting with Mercs. I'll let MadamEnigma explain it to you. Not a very good TA expert, although I have tried and managed the set before.
Mercs/Pain, the powerful tanking build. Aside from heals and buffs you won't do much damage against more powerful foes with higher resistances, but against normal enemies prepare to slaughter them with very high damage buffs for your pets (especially if you take assault). I can't even begin to explain what a Commando with Serum and Painbringer can take. He becomes a walking AV for a minute.
I have a /storm MM. NONE of my abilities is slotted for end reduc. I can run Snowstorm, Hurricane, Tactics, and (insert the group stealth from storm lol) and I have zero problems with endurance. Currently 37 though so not sure how its gonna be at higher level.
I found Merc/Dark to be incredibly good as a pairing. The -res helps offset the (overstated, IMO) lethal resists in the late game, Fearsome means the mob ain't runnin' from the AOE deathcloud the mercs will drop, and you've got a really, really nice heal to keep those crazy boys on their feet and shooting. 
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I'm loving my Mercs/TA. It has an odd synergy- several of /TA's powers are "stay away from me" powers. Ice Arrow is a straight-up Hold, Glue Arrow is a powerful Slow, Oil Slick Arrow is a slow that can also do ungodly damage if you light it on fire (which the Commando can do for you).
The reason /TA works better with Mercs than with any other set is, actually, because of Oil Slick Arrow. There's a bug that has pets uncontrollably run away when they're in it and it gets set on fire. It doesn't actually hurt them, so it's a bug. Luckily for you, Mercs are 100% ranged pets, and therefore have no need to step on the burning oil. It's one of the most evil things you can do. Think about it- if you've fully debuffed them, the enemies are:
-Being burned by acid
-Blind
-Choking on toxic gas
-Arms glued to their sides
-Being repeatedly concussed
-Covered in oil that happens to be ON FIRE
-And being shot by mercenaries
...Yeah. It's awesome.
The only bad thing I'll say about TA is that it has no healing arrow. Again, luckily for you, you've got a Medic in those Soldiers of yours. He isn't that great, really, so you may want to pick up Aid Other. Or not. If you team a lot, it won't matter so much.
I'd say something but out of fear of the Trick Arrow forum syndicate i'll remain silent...
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Okay, being an Altaholic... and something of a masochist, I've decided to roll up yet another Mastermind. This time, I will be revisiting Mercs, which I never got past 10 the first time around. (Not to do with the Mercs themselves - bad choice of a secondary...)
So... I'm trying to decide what would be a good compliment to Mercs. I already know /Force Fields is out (the aforementioned bad secondary - I fell asleep playing that one!)
I'd also rather avoid /Poison and /Traps, as I have one of each of those currently.
So... all that remains is /Dark Miasma, /Storm Summoning, /Pain Domination, and /Trick Arrow... how do each of these fit with Mercs?
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I am a huge fan of Merc and Merc/Storm is by far the nastiest combo. Reasons?
1. Well, all of them are ranged so you can just stand in between them with Hurrican running. No melee can get close!
2. Freezing Rain is so awesome!
3. Put 4 pet uniques in Tornado... your pets will be having close to 47% resistance to smashing/lethal and about 41% resistance to Fire/Energy/Cold (21% from Steamy and 20% from uniques).
I am not saying Merc/Storm is the best but it's pretty close! The only thing I have trouble with is AV with controls/psi damage.
What's left is to normalize all Assassin Strikes and improve Stalker's old sets (Claw, MA and EM)! You don't need to bring back the missing PbAoE attack. You just need to make the existing ones better! For example, make Slice a WIDER and LONGER cone.
It really depends all on your playstyle. Each of those secondaries works very well with mercs, so it's going to end up being your personal preference that decides which you'd like to go with.
Storm is good for knockback and debuffs, pain focuses mainly on buffing and healing, TA is debuff-heavy with some control, and dark is a mixed bag. My suggestion: try them all out and see which you like best,
"If I had Force powers, vacuum or not my cape/clothes/hair would always be blowing in the Dramatic Wind." - Tenzhi
Characters
My Merc/Pain was a lot of fun and is a powerful combination. -res proc in your pets and you're pretty much set. I personally took my patron immb to keep enemies at range.
My Merc/FF is a pretty effective toon. Since they're all ranged it's not hard to keep them in the bubble, and FF is good at keeping foes away. Course there's the bubbling everyone every 5 minutes (you don't HAVE to, but why wouldn't you?) which some people hate, but I'm used to it- having your pets be more durable means you don't have to rely on Bodyguard Mode so much, and can have more control over what your pets shoot.
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My Merc/FF is a pretty effective toon. Since they're all ranged it's not hard to keep them in the bubble, and FF is good at keeping foes away. Course there's the bubbling everyone every 5 minutes (you don't HAVE to, but why wouldn't you?) which some people hate, but I'm used to it- having your pets be more durable means you don't have to rely on Bodyguard Mode so much, and can have more control over what your pets shoot.
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No.
I tried this combo once. I got to level 10, and deleted it because of how dull it was - the first and only time I've fallen asleep playing the game!
It may have become a workable set, if I had gotten to Repulsion Bomb and Force Bubble... but I couldn't.
I will never take another /Force Field Master Mind. I need to DO something when playing, not just sit and watch my pets do things.
I may go ahead and try /Trick Arrow... I've never really played that one before...
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I kind of feel the way OneWhoBinds does. My 50 Robot/FF MM is a powerhouse. But boring. A more active secondary for MM is what keeps me hooked, hence why I actually prefer my Mercs/Dark.
53 Bots/FF/Mace Mastermind | 53 NRG/FF/Electricity Defender | 50 Time/Dual Pistols/Soul Defender | 50 Demons/FF/Mace Mastermind | 51 Necro/Dark/Soul Mastermind | 50 Thugs/Time/Mace Mastermind | 50 Ice/Ice/Arctic Tanker | 50 Plant/Rad/Earth Controller | 50 Illusion/Trick Arrow Controller | 50 Gravity/Force Field Controller
Yes, I like Force Fields.
I got to this topic a bit late (damn forum passwords and my bad memory) but I'll still throw this out there. I really like my Merc/Pain because unlike some other secondaries, Merc KB is no issue....it is just a form of soft control. Team's love you and you make for a hell of a tankermind. If you care....the sound a full squad of Mercs makes is awesome too....it sound's like a bad day in Fallujah.
Okay, being an Altaholic... and something of a masochist, I've decided to roll up yet another Mastermind. This time, I will be revisiting Mercs, which I never got past 10 the first time around. (Not to do with the Mercs themselves - bad choice of a secondary...)
So... I'm trying to decide what would be a good compliment to Mercs. I already know /Force Fields is out (the aforementioned bad secondary - I fell asleep playing that one!)
I'd also rather avoid /Poison and /Traps, as I have one of each of those currently.
So... all that remains is /Dark Miasma, /Storm Summoning, /Pain Domination, and /Trick Arrow... how do each of these fit with Mercs?
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