You want control more than damage to solo Rancor. Especially in P1 where it's tricky.
1. TFP should open with basic and crit the Boss to put Tenacity Down asap. (Retreat and try again if you did not crit on first move, it's not GW so you can change the result to crit within a few tries)
2. Vader slow on first move, and no using of culling blade except on the side guards. (Culling blade removes all debuffs including Tenacity Down and Slow and may take a while to put them back up. If you REALLY REALLY want to use culling blade, make sure CD on Force Crush is up before culling blade)
3. EP has 2 AOE that can help reduce TM on the side guards, the AOE stun as well as the heal.
4. Leave the side guards alone as they will naturally die from all the AOE damage but while they are alive and debuffed, they help your TFP to gain TM faster.
5. Rex gives TM with Squad Discipline whether you are debuffed or not, I always use this whenever CD is up.
6. QGJ TMR is not always reliable, you want consistent TMR from effects such as Rex basic.
Once you've gotten past P1, you just need to give Rancor Tenacity down and Slow(at start of P2, P3, P4), then set to Auto with Basic attacks only. This doesn't go that slowly as there's 4x speed now. Saw someone solo Rancor with just Zader, TFP, Rex. Good luck have fun.
TLDR: Control>Damage, Get Rex.