Ok, this problem originally cropped up when I faced Balthazar at the monastery. I didn't know the exact reason but the fight soon froze after initiation. My work around at that point was to simply drop him in seconds flat before he activated anything. The problem recurs in the final Ascension fight when Mel spawns the Five. Game freezes when he activates Lunar Stance it so happens.

So to troubleshoot this I tried to isolate the problem. I've gone through all the permutations I can think of (Ascension before fixpack, Ascension after g3 fixpack, etc), removing all other mods such as UB, tactics, wheels of prophecy, you name it, and it all boils down to SCS II. No special component of it is even required, it's simply the basic "Initialise Mod" component which is needed to screw up the Lunar Stance. If I just remove SCS II from the equation everything works perfectly.

I can't post this in G3 forums because it's down so I thought I would see if anybody else experienced this. I'm a bit perplexed as to how such a thing could go under the radar. There was a post in G3 forums Oversight subsection about the oversight added Monk HLAs freezing the game but I never used that mod. So it seems the problem may not be with Oversight after all.

As for my current WEIDU log it's pretty bare, and you can mix and match any of the fixpack components without affecting the bug, and you can still experience the bug in the monastery fight if you remove Ascension entirely:

~BG2FIXPACK/SETUP-BG2FIXPACK.TP2~ #0 #0 // BG2 Fixpack - Core Fixes: v9.01
~BG2FIXPACK/SETUP-BG2FIXPACK.TP2~ #0 #1000 // BG2 Fixpack - Game Text Update -> GTU Light (by Wisp): v9.01
~BG2FIXPACK/SETUP-BG2FIXPACK.TP2~ #0 #3 // BETA Core Fixes (please check the readme!): v9.01
~BG2FIXPACK/SETUP-BG2FIXPACK.TP2~ #0 #101 // Improved Spell Animations: v9.01
~BG2FIXPACK/SETUP-BG2FIXPACK.TP2~ #0 #108 // Remove Second Attribute Bonus for Evil Path in Wrath Hell Trial: v9.01
~BG2FIXPACK/SETUP-BG2FIXPACK.TP2~ #0 #109 // Corrected Summoned Demon Behavior: v9.01
~BG2FIXPACK/SETUP-BG2FIXPACK.TP2~ #0 #110 // Additional Script Fixes: v9.01
~BG2FIXPACK/SETUP-BG2FIXPACK.TP2~ #0 #111 // Bard Song Fixes: v9.01
~BG2FIXPACK/SETUP-BG2FIXPACK.TP2~ #0 #112 // Wizard Slayers Cause Miscast Magic on Ranged Attacks: v9.01
~BG2FIXPACK/SETUP-BG2FIXPACK.TP2~ #0 #113 // Additional Alignment Fixes: v9.01
~SETUP-ASCENSION.TP2~ #0 #0 // Ascension v1.41 (requires ToB)
~SETUP-ASCENSION.TP2~ #0 #1 // Tougher Abazigal (optional, requires ToB)
~SETUP-ASCENSION.TP2~ #0 #2 // Original Tougher Demogorgon (optional, requires ToB)
~SETUP-ASCENSION.TP2~ #0 #3 // Tougher Gromnir (optional, requires ToB)
~SETUP-ASCENSION.TP2~ #0 #4 // Tougher Illasera (optional, requires ToB)
~SETUP-ASCENSION.TP2~ #0 #5 // Tougher Yaga-Shura (optional, requires ToB)
~SCSII/SETUP-SCSII.TP2~ #0 #1000 // Initialise mod (all other components require this): v19
~WIDESCREEN/WIDESCREEN.TP2~ #0 #0 // Widescreen Mod -> for the original Infinity Engine (CHOOSE THIS!): Widescreen Mod v3.02