Here is some details of the showdown of between Luke and UnuThul:
Luke reached out in the Force and found Raynar reaching back,
descending rapidly from the command deck atop the bridge structure.
Raynar's presence was as murky and heavy as always, and as soon as Luke felt it, it began to press down inside, urging him to turn back.
Luke did not resist. He was going to leave, he wanted to leave . .
. with Raynar. Luke began to exert his own will, pulling Raynar toward him, using Raynar's own power against him by binding their presences together with memories from their past: of how Luke had once helped protect Raynar's family from the Diversity Alliance, and how he had later helped Raynar's father destroy a terrible virus that could have caused a galaxywide plague. They were going to leave together. UnuThul wished Luke to go, Luke wished UnuThul to go with him, and so they would go together.
UnuThul wished it.
The weight inside suddenly diminished as Raynar started to retreat.
Luke tried to stop him, to find some part of his former student that he could hold on to. But UnuThul still had the power of the Colony behind him, and he called on that power to break the bonds of remembrance the Jedi Master had so quickly woven. His murky presence wrenched free, and the heaviness vanished from inside Luke's chest.
The highlighted part makes it clear that Luke has history with UnuThul (his former student) and makes him realize his past in which Luke helped his family a lot. This sounds like Dun Moch and it works because of Luke's history with UnuThul.
He tries similar approach again. In the final engagement between the two, Luke uses his strength in the Force to choke UnuThul and simultaneously make him realize through Dun Moch that the killicks would devour their own worlds and destroy themselves. But UnuThul is not easily convinced and fells unconscious.
UnuThul has been described like this:
The pain of losing an arm might have forced a common Jedi to stop fighting, but Raynar was no common Jedi. He had the Force potential of the Colony to draw on, and he did that now, swinging his remaining hand up to hurl Luke down the corridor as he had done before.
Yes, this is feat. He draws power from that Colony to continue fighting.
However, UnuThul never displays any incredible Force powers against Luke. At best, he performs a Force push which does sends Luke packing but his guard is down at that moment. Later on, UnuThul attempts to deal with Luke on the basis of his skills with the Lightsaber. A foolish move and display of naivety. As pointed out here;
Awed by their own strength, they so often believed strength was the answer to every problem. Luke was older and wiser. While Raynar swung, he pivoted.
Funny thing is that how easy it has become to destroy the MYTH surrounding the Luke Skywalker and the power of his opponents. Luke worshipping at its peak. 🙄
This duel has nothing on Sith Emperor, whose experience and command of the Force is incredible. His mental assault is completely different in nature and on a whole new level. His dark side powers are overwhelming and rip through the defences of even extremely powerful Jedi like butter.