From what i've seen,here my thought:
-Michael Myers:
+Possession:This gonna be the main problem for Xefro because i don't see him have anything to counter this.
+Immortality and Regen:I can say that Xefros gonna have a hard time putting Michael down due to he is extremly hard to kill by normal mean.Regen might help Michael a bit in a long battle.
+Stealth skills:This pretty much one of Michael biggest adventage,if his opponet don't have anything to bypass that,Michael can sneak up and process to hack them to death.
+Illusion Creation:As you said Xefros had encounter illusion user before so this is useless.Unless Michael use Illusion Creation + Stealth skills combo to trick him then Xefros have no trouble dealing with Michael illusion.
+Intelligence:Michael is very smart for a slasher movie standard,i can say,he can trick Xefros by playing dead to slay him,he mastered the Cults mental manipulation and can use it to intimidate his targets,he also is a extremely calculative killer capble to using his surroundings as his advantage.
In Xefros profile,i seen his weakness:"Is very trusting of anyone who shows even the slightest amount of kindness to him".This properly gonna be one of his downfall,because Michael have:
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Capitalizing off of Laurie's fears of killing another innocent, Michael fakes like he's someone different, only to kill her once she draws near
-Xefros Tritoh:
+"Xefros live in a planet where 'kill or be killed' but he manage to survived to the age of 14 and joined a revolution to overthrow the government":This mean Xefros has face with many people that is much worse so Michael ain't a big deal with him.
+Pain Inhibition/ Resistance to pain:Both of them pretty much the same,i can't tell the different.
That all i can think.