He has an unblockable attack (not even Perfect Guard of Heavy Stander will work). Battle is solo only. I'm on practice mode because I had already killed him for today (1 victory per day).
-You can fall off the perimeter of the map and instantly kill yourself, so keep to the middle at all times
-Muir can punch away the walls of the stage. While this exposes more places for you too fall from, this also allows you to hang from more places and exposes more area necessary to finish him off with (more on those later)
Phase 1
-Easiest phase. Slow attacks and nothing that is unblockable
-Single punch with his right arm
-Single punch with right arm to the ground, creating a straight shockwave. Must be blocked by Perfect Guard or Heavy Stander
-Two hit combo
-Side attack that hits twice
-After his HP drops to a certain point (8 bars in Normal), he will initiate an AoE that will instantly kill you. At this point, you need to head to the edges of the stage to where the red arrows are and press E (after waiting for the square icon to appear, do not spam it) to hang from it.
-While hanging from it, you will do a teeter-totter minigame. Tilt the teeter-totter with left and right movement buttons (A and D on keyboard and mouse mode) to roll the ball to the blue outlines while preventing it from falling off the sides. If it falls off, the minigame will fail immediately and you will climb back up with reduced HP. The more wins you have with the minigame, the more HP you will retain when you climb up, up to a maximum of what you had before you hanged (i.e. it won't heal you).
Phase 2
-Initiates after the first AoE
-Single punch with right arm
-Single punch with right arm to the ground, creating a straight shockwave. Must be blocked by Perfect Guard or Heavy Stander
-Side attack that hits twice
-(2:36)Two hit combo has been upgraded to three hit, with the third hit being completely unblockable. Unfortunately, he does this move way too often to rely on SP. Block the first two hits then dodge, by sprint smashing or rolling, to YOUR left (HIS right) to avoid the third punch. Once you get good at this, you don't even need to use a dodging maneuver; just run to the left and begin winding up for a 4 hit combo.
-Spits out purple poison, leaving behind a gas that stays for a while and does DoT. He probably won't do this too often if you keep near him.
-Roars, blurring the screen if you have motion blur on, and does a 5-hit combo. The first 5 hits can be blocked with anything, but the final hit must be guarded with a Perfect Guard or Heavy Stander
-Occasionally does AoE again. Hang like before.
-Keep yourself either at the middle of him or slightly to the left (his right) so that you can get ready to dodge the third hit of his three-hit combo
Phase 3
-Initiates when the game tells you to attack him from behind. Attacking his back enough, probably to accumulate knockdown, will cause him to be knocked forward.
-Attacks are identical to Phase 2
-The game tells you that you can knock him to the edge of the stage for an "early victory". Unfortunately, this is the only way to finish him off as, after a while, he starts taking 1 damage. Knocking him off will be easier if you had let him destroy more of the walls. I got lucky in the video and managed to finish him off almost as soon as the phase started, but I've had a run where it took a while.
And yes, I didn't use Shield Deflection or Focal Point at all because I literally recorded this on the first day I played Vindictus in a long time, so my playstyle was very "old".