Shouldn't be Act 3, should be act 5. Sorry about that. I'm not going to use a Prayer merc. Those aren't necessary for MFing IMO. Meditation alone would be enough to boost Life/Mana without the need for Prayer. Even then, I wouldn't use Insight anyway since this Sorc will have so much Mana Regen % that Meditation would just be rather pointless anyway. The merc I'm going for is more of a power-house tank that just simply owns things that aren't getting hit by Fire Ball and Frozen Orb and that are Fire and Cold Immune. So Insight and Prayer don't really fit for that kind of goal. Although, it's an idea and it isn't like I can't redo a Merc whenever I feel like it. =P
I could go with Infinity, Dream, with Fortitude armor and make it a Defiance or Holy Freeze merc. Then again I could run an Act 5 Stun merc that uses Last Wish Eth CBlade/BOTH Eth CBlade/Death Eth CBlade, Fortitude armor, with a Cham'd and Pul'd Giant Skull.
I think that would be some good power-house mercs that can fight for themself while I'm picking crap up and looking at stuff while monsters are moving around. The only two mercs I like are Act 2 and 5.
Firstly: Insight doesn't regen life on it's own. It synergises with the merc's natural Prayer aura, effectively providing twice the normal regen.
Even with 1 point in Warmth, +mana items and Magefists, and no points in Energy, you're going to run out of mana fairly quickly spamming Fire Balls at 105% FCR (8 frames). Insight makes you almost immune to mana burn, allowing you to teleport to safety if you're surrounded.
It's also a pretty decent whack-stick on it's own. The only dual Cold/Fire immunes you'll come across are Bosses, and even then their minions aren't dual immune, you can clear them, then use Static then Telekinesis while your merc kills the remaining boss.
An Act 5 merc doesn't need Giant Skull, he's already got knockback in the form of Bash. Even then, knockback messes with your FB aiming, which aint a good thing. If it's a dual immune, then Telekinesis provides enough knockback to keep them in place. Pul is possibly the worst thing you could socket it with, the small defense bonus is negligable on a helm.
Andariel's Visage fits a merc better than a GSkull does. You'll want some leech on your merc, Andy's has up to 10%, as well as other great mods for mercs.
If you can afford it, then Infinity, Fortitude and Andy's Visage on a Holy Freeze merc will probably serve you best. Don't bother with Dream, I don't know why people suggest it as a merc helm, it's a very average merc hat. Leave Dream for the Pallys/Sorcs.