So first thing's first with DM/Inv on Scrapper or Brute:
You'll get more survivability from the Brute variant with the potential for higher Resists. Brutes have the same Resist modifiers as Scrappers, but their Resist hardcap is higher, so external buffs and things like Unstoppable will yield better results. Brutes have a higher HP modifier, which means you'll also have more survivability from a higher base HP and even higher HP with Dull Pain than on a Scrapper. Fury is trivially easy to get and maintain now, but getting to the upper end of the bar and maintaining that is much more difficult now. A Brute may or may not fit your gameplay style or not be thematically in line with the kind of character you want to make.
Scrappers benefit far more from +Damage because of their higher Melee Damage modifier, so getting as close to perma-Soul Drain as possible is best if you choose this variant. A Scrapper has a lower HP cap - the silver lining to that being HP Accolades and perma-Dull Pain means you do not need to devote any IO set bonuses to +HP. Scrappers have Critical, so if you enjoy seeing double damage on your attacks every once in a while, or are used to Criticals from other MMOs then that may be more appealing to you. A Scrapper may or may not fit your gameplay style or not be thematically in line with the kind of character you want to make.
For my DM/Inv and every other combination I've made, I've usually picked Scrapper. I like that I consistently do high damage and could snap at any moment, delivering double the damage without any warning. It's in the theme of my characters and the way I like to play.
Now for the Defense and softcapping some or all of the Typed Defense (except Psi):
Softcapping Smashing/Lethal with one foe in Melee is very powerful, but newer content is throwing out more damage types - specifically Energy and some Fire damage as well. Both of which usually have Debuffs attached to them, such as -Recharge on the Fire attacks. It is important to softcap Smashing/Lethal/Fire/Cold/Energy/Negative if you are going for a top tier build. It's both very possible and you don't have to give up much at all to do so on a DM/Inv. It will be costly, but you will be a living example of your Secondary set's name: Invulnerability.
The kind of layered damage mitigation you can achieve on a DM/Inv is staggering. -ToHit on enemies with your Primary attacks. Two Heals - one a small Heal that is constantly topping off your Health and the other nearly a full Heal that can also permanently boost your max HP. Around 73% Smashing/Lethal Resist and 26% to 28% to Fire/Cold/Energy/Negative on top tier builds. Last but not least, softcapped Typed Damage (except Psi) with only one Melee foe. All of those in combination provide an astounding amount of survivability, and this is before adding the Destiny Rebirth Radial power, which can give +Regeneration permanently; 200% Regeneration at it's lowest. I would be hard pressed to try and obtain 200% Regeneration with IOs without sacrificing many things in my current build.
With my goal on my DM/Inv I wanted to make a character that is incredibly hard to kill, but without giving up much on the offense. The build has perma-Hasten and is nearly perma-Soul Drain. At minimum I always have 60% extra Damage and 12% ToHit. Much more if I am playing in my preferred playstyle - jumping into huge groups of enemies. I wanted to also shore up Dark Melee's lack of quickly available AoE attacks (I hate the long animation time and small cone of Shadow Maul) so I grabbed Fire Ball. It fit the character theme and helps my build's performance as well - perfect! Adding in the Pyro Judgement further helps my build's primarily single target focus.
It took several pages in my DM/Inv thread, but together the Scrapper community and I hashed out a DM/Inv build that is really quite exceptional. Until new IOs arrive on the scene, there's nothing I'd change about the build except to adjust to differing playstyles - and that is just what some players have done with it!
Hero Plan by Mids' Hero Designer 1.942
http://www.cohplanner.com/
Click this DataLink to open the build!
Succubus Kali: Level 50 Magic Scrapper
Primary Power Set: Dark Melee
Secondary Power Set: Invulnerability
Power Pool: Fighting
Power Pool: Speed
Power Pool: Leadership
Power Pool: Leaping
Ancillary Pool: Blaze Mastery
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