Invuln is based off typed defense, so trying to build up positional defense is a waste. The two don't stack - only the highest value is used. Instead, try to build up your typed def as much as you can.
As for how Invincibility works, it grants two defense bonuses per mob in range:
*) +3.75% s/l/e/ne/f/c def (5.85% slotted)
*) +0.75% s/l/e/ne/f/c def (1.17% slotted)
The first one (3.75%) doesn't stack, so you can only have one of them (this is the "first target bonus"). The second (0.75%) does stack and will stack up to 10 times with 10 mobs in range. There is an extra wrinkle in that it will briefly double stack for a split second, but it's not something to rely upon.
tl;dr:
+4.5% for one mob (7.02% slotted)
+0.75% for every mob after that (1.17% slotted)
I am not good with numbers. But I run a Ss/Invuln as my main farming build. I can run the wall in Cim pretty much without stopping except when Weave drains my end. Invuln is an awsome set, not as good as Wp or Sr, but when you can click heal for over 1k, you realy cant beat it. Its a very tough set, I farm Lib and 54 lvl boss farms in AE for my VG. Sure, I eat Endurance for a snack, but since you have Dm/ you sholdnt be bothered with that too much.
Sory I cant post numbers or a build for you (Lost my mids build somehow), but its pretty straight forward of a secondary. stay away from Unstopable tho, I dont care what anyone says, the crash from that IS a killer if you arent out of combat...No matter what I tried with that power, greens and blues wont help during combat when that drops, so I respeced out of it.
Anyway, Good luck with Invuln, its a very fun set and you will live almost as much with SO's as a IOed out Wp (trust me, watched my buddies Ss/Wp die a few times horrible deaths while I clung on to life and died shortly afterwards MUHAHAHAHAHAHAHAHAHa But with Dm you have an extra heal and you definately want to slot Stam Big time if your going to take weave.
Thats my 2inf worth. Enjoy
I hate all this terrorist business. I used to love the days when you could look at an unattended bag on a train or bus and think to yourself.... I'm going to take that.
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Invuln is based off typed defense, so trying to build up positional defense is a waste. The two don't stack - only the highest value is used. Instead, try to build up your typed def as much as you can.
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I got my wires crossed there, I meant typed defense... TY for the correction. Those are some decent #'s Sarrate, thanks for posting that. And thanks for the insight on unstoppable Ragnar, funny that you happened to mention that as I was wondering if I should go unstoppable or darkest night. DN is always available but sucks end like a black hole. Unstoppable (slotted) looks like 80-90% resists vs everything but psi + a massive end regen boost but with a serious crash at the end. I'll have to experiment some on test and see if either fit my playstyle.
Thanks again guys 
I just spent about 600 million soft capping my Invul brute and let me tell you IT IS AMAZING. people have even noticed as im pulling two groups of all level 54 bosses and watching them swing and miss much like a granite brute/tank..take some time and look into the best things to get your defense up in smashing/lethal and the other elements its worth it!
Nice to hear Mathias. I've been tinkering with mids and can get S/L defenses capped with one enemy, but my other defenses suffer. In my dream build I've gotten what I think is a decent balance of defenses/resistances/ haste (defenses: type 42% s/l, 38% c/f, 37% en/ ne, positional 23% ranged and 21% melee, resistances s/l 71%, f/c 23%, toxic 23%, 12% en/ ne; 65% haste), this should cap me with enough critters. I chose unstoppable for the hard targets and skipped on darkest night (prob a big no-no, but not sure my end would hold up with the number of toggles im running).
Been a fun ride so far to lvl 32, see how it goes once i meet my first dark ring mistress...
Hey guys, just made my first brute after all these years and figured I'd go "old school" and start out with invulnerability. I ran to mids and plugged in a rough lvl 50 build to see what I can squeeze out of it. The build looked okay, with @ low 30ish position defenses and almost perma-dull pain. What I am unsure of is how invincibility actually works: I know you get a defense buff based on how many critters are around you, does anyone have an idea of what the percentage is per mob? Since I solo alot I'd just like an idea of how much this power will actully improve my survival is all.
Edit: primary is DM btw.