DRG has 2 combos. A dot combo and a damage combo. The basic concept is to use blood of the Dragon and then do the 5 hit Chaos Thrust combo chain, then the full Thrust combo chain and alternate them. Use your jumps as available. When you've done 2 mirage dives (proc from jump) your next Geirskogul will enable you to enter life of the Dragon state. In this state, she can use Nastrond every 10 seconds for up to 30 seconds (depending on duration of blood of the dragons the time). This is the most basic rotation.

True Thrust
Disembowel
Chaos thrust
Wheeling thrust
Fang and Claw
True thrust
Vorpal thrust
Full thrust
Fang and claw
Wheeling thrust
(start from top of life)

Weave in jump, spineshatter dive, Geirskogul, mirage dive, Dragonfire dive as she sees fit.