I want to make an invul scrapper but...

I am worried about the lack of psi protection.

Is it a dealbreaker for Invul?

I fear carnies and arachnos will eat it alive.

Also what is the best primary to go with invul?

By no means is it a deal breaker, but it can be a thorn sometimes.

Carnies just be a little careful, pop some purples, rely on the team a little more, keep Dull Pain ready are all ways to survive. Arachnos just make sure the first things you fight and kill are the Psi ones and you will be fine.

I found DM to be a fantastic pairing, but Invul is so strong in its own right that it doesn't really matter.

Also, learn Target Priority when assaulting said packs. Really, there isn't that much psychic in a pack of Arachnos. But there is a serious variety of exotic energies in those groups.

If you've got the Inf to softcap your S/L/F/C/E/N Defenses you usually get around 25% for Melee/Ranged/AoE Defense. Certainly helps and with the non-positional Psi damage out there, you always have Dull Pain to put you at the HP cap; Siphon Life to constantly top off your HP if you have Dark Melee too. I have the Accolade power Elusive Mind as well and make use of it on the Psi AV battles. Very helpful!

Pair Invul with a primary that has strong Knockdown/Up controls like Broadsword or Katana. When you run into something that is dealing a lot of psi damage cycle your knockdown attacks on them. If you go with a set without that type of control consider picking up Air Superiority to fill that soft control gap.

A big majority of the Psi attacks coming your way are going to be typed psychic and ranged. Building you Eng/Neg Eng IO set bonuses will help you against the ranged psi attacks. You can grab 4 Erads pretty cheap and get a nice +hp boost and 3.13% Eng defense.

My first character was a Claws/Invuln Scrapper, and I still have fun with them, even though I haven't bothered to really IO him out. I find in general that when I'm going at a reasonable pace, Dull Pain is recharged each time my health gets really low. Combined with not being stingy on Unstoppable when I need it, taking out problem enemies first, and using my soft controls (alternating Focus and Shockwave knockdown/back loop on nasty bosses ftw!), I find /Invuln to be quite nice for Scrappers.

Well, to be honest, I find taking out nasty enemies and using soft/hard controls to be a good idea on absolutely any character. So really, it doesn't seem that different to me.

I have a MA/Inv, that is by far my favorite toon to play, I haven't really thought much about other alts since and was probably my quickest 50.

But anyway, I have him softcapped to all types, except Psi, and as said above doing that gives you some decent positionals, so I would say if you are also running CJ/Hover, and Weave, maybe Maneuvers too, it wouldn't be that big of a problem. Dull Pain for me takes me to the Scrapper HP cap (don't know if that is saying much) and only has 10 secs of downtime.

DM/Inv is the most survivable scrapper combination, if you build it right.

I wouldn't worry about psi.