There's a discord server called the "HSTR Project" that has lots of valuable information.
Real quick RJT summary:
1. Try to get your first Illuminated Destiny before Nihilus' first turn. Do this by inflicting speed down and Expose on him asap, and then using specials on the adds to keep reducing his turn meter while you reduce ID's cooldown.
2. Resistance Trooper & Scavenger Rey heal themselves through damage. Having Trooper at g12 with a Multi-tool is vital to give him 10% Health Steal.
3. RJT heals herself a bit through health steal, and also through her constant foresight. You have Insight zeta'ed, so that's good.
4. BB8 and R2 are the hardest to keep alive - applying Self Preservation Protocol on BB8 can significantly help with that by letting his dodges heal himself and R2. Additionally, having them at g12 with Multi-tools can let them recover a small amount of health when they attack.
5. Use specials as frequently as possible. Trooper and Scav should only use basics if their specials are on cooldown (which should be very rare). More specials = more ID. More ID = more damage/score and healing.
6. Use foresight to protect your team from Nihilus' Drain Force - I set up my squad so R2 has foresight from RJT and BB8 has foresight from Smokescreen on the first Drain Force. Future ones require more watchfulness, but it's much easier to predict now. Since those two are the ones who need the most healing, watch their health more. The only exception is when another character was recently annihilated, and is almost dead.