A quick question
Necro/Dark is a classic power pairing. I am not sure what trouble you are having, but necros are inherently more squishy than demons.
I have a level 50 Demons/Thermal, and he is very durable. The thermal shields stack nicely with the ember demons aura, and the thermal heals mesh well with the Demon summoning heals.
I started a Demon Summoning/Sonic resonance mastermind for thematic reasons with the new issue. I am pleasantly surprised how durable and effective the pairing is with the lower level PvE content. I find it more durable than my Bots/Time Manipulation -- so far at level 27.
Triumphant Defenders Forever
Psylenz FF/Psi, ArticQuark Storm/Rad, Symon BarSisyphus Bots/psn, Max VanSydow Thugs/Dk, Cyclone Symon Bots/stm, Blue Loki Ice/Cd, Widow 46526
HelinCarnate:OMG it is so terrible. I have the option to take 3 more powers but no additional slots. Boo F'ing hoo.
Would it be better? Dependant on what you want to do and your budget. Another factor is what sets do you like to play. There are a couple posts on the most recent two pages concerning necro / dark builds, as well as different combinations of demons.
What seems to be giving you trouble?
Demons are just the best pets, especially for survivability. They're great even with only SOs.
I definitely recommend Demons for a Mastermind. I'm using Demons every time now, when I used to use Robots exclusively(and I recommend Robots as well, but not as much as Demons).
Demons/Time is pretty good, but I feel Demons/Force Field and Demons/Sonic are better, and certainly easier than Time.
Ive played a Necro/dark MM to 50 and enjoy the AT but I find the zombies annoyingly squishy.
Would a demon/ x be a better combi and if so what would be the x (I dont really fancy TA btw)