Rune King Thor. Would be a fun fight to watch but i don't see RKT breaking much of a sweat and dispatching him maybe not as casually as other but still dispatching him. RKT seems to be working on a higher mystical level then other magic user. I include Dr. Strange, other Skyfathers and the various major demons like Zom, Dormamu, Satanish and Mephisto.
Mordu might not be killable but should be no trouble for RKT to either contain or fully drain him of powers. RKT brings too much for this fight. He'd know the complete past of Mordru, what he was thinking, what motivated him, what his weaknesses are or were. He'd know what he was upto in the present and be able to change Mordu's future.
Mordu is a powerhouse there's no doubt but he seems to have been defeated over and over again. Drained by Darkseid, the sorcerers of Zerox stipped him of power, he gets contained and has a fear of containment which RKT would know during the fight. Basically i just see RKT working at a higher level then any other mystic. Here's a question. Do we even know if RKT can even be harmed by a physical or mystical attack?