Depends on your play style of course but the old faithfulls are good; leadership pool powers, Aid Other/Aid Self, the two ally bubbles, the mez resist bubble and the personal forcefield.
I've not properly tested Serum and reading about it people are rather mixed on it so I'd move your MM onto the test server first and have a play about with it. The AoE blast doesn't appeal to me as I've already got more reliable knockback powers in /FF but with IO sets you may find uses.
I'm a bit different and actually like the /FF powers, Repulsion Field, Repulsion Bomb, Detention Field and Force Bubble but again I'd try em first on test as they are very very situational powers.
Force bubble works quite good with mercs, as 99% they only do ranged damage. Though with few downsides, at higher lvl you might face quite some mobs having high S/L resist and without debuffs it can be annoying taking them down with mercs.
Serum is kinda funny, it makes 1 of your pets sort of Force of Nature power (blaster epic), high resist with mez and nice recovery. But as your pets prefer ranged and cone, having 1 in melee wont help much tbh since they cant taunt or something. (example, using your commando to tank a EB).
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All I can say is that whenever I've seen a Forcefield Mastermind, I'm always super jealous of the containment field FF power. If only because there will be a mob with two bosses then suddenly one of them can't touch us. It's a nice surprise and seriously helps trim the doom in particularly troublesome mobs.
That is assuming it's slotted for accuracy something crazy...or so I've been told.
Oh and Force Bubble? If you get that (and it's awesome) be prepared to be told people just can't team with you if you can't use it selectively. We had to fight wailers in an office building with a FF Mastermind with it and....well....none of the melee toons could CATCH any of the damn wailers because the mastermind kept moving and all the enemies would be suddenly repelled in another direction. It's a great power that I wish I had, but definitely not something you should have on all the time as this guy did.
I've recently made a new MM with mercs and force fields. Are there any powers from either set people would suggest avoiding and why? What other powerpools do you reckon would help me make the most of this build?
cheers for any replies.