G10+ with speed about 230+.
First time I did it was right after I unlocked him.
I had G11+0, +94 speed at the time, zzCLS (Learn Control and IBAT). His leadership zeta makes it slightly easier as he'll gain more TM if the Rancor resists, which it will...)
I know most people say IBAT and Leadership but I found Learn Control exceptionally helpful because Luke will resist almost anything when he's not in Call to Action.
I didn't try lower but that's because I had saved up to take him right to G11. CD mods with CD triangle and offense cross. He really doesn't need a CC set but they had great speed secondaries.
Phase 4 is tricky because it seems like the Rancor is taking tons of turns but someone described a glitch where each turn shows double. He was at "0" on the enrage counter for several turns without actually enraging.