A few years ago, I did almost exactly the same thing:
14 Modular abilities
+ 0 Human Form
-3 True Shapeshifting (can only turn into "living creatures")
-1 Supernatural sense(psychic): shapeshifters
-1 Pack instincts (any shapeshifter dominated)
-3 Domination (ranged) Plot device, not a combat device.
-6 mythic recovery
-6 Mythic Toughness
-6 Physical Immunity: Own powers, Requires at least one Mental consequence before it can be hurt
Catch Holy Stuff, silver, wolfsbane: +2
Total refresh: 38
+6 Weapons
+5 Endurance, Fists
+4, Alertness, Stealth
+3 Conviction, Athletics
+2 Survival, Might, Discipline
+1 Presense, Scholarship, Lore
-2 breath weapon Power=Endurance Control=Weapons (grapple)
-2 Claws (venomous)
-1 echoes of the beast(tremorsense)
-1 Spider walk
-4 Supernatural speed
-2 Inhuman Str
+6 Might
+5 Endurance , Fists
+4 , Alertness, Athletics
+3 Conviction, Intimidation,
+2 Survival, Weapon, Discipline
+1 Presense, Scholarship, Lore
-2 Hulking size
-6 Mythic STR
-2 Stomp Power=Might Control=Endurance (maneuver: knocked over)
-1 Claws
-1 echoes of the beast(hearing)
+6 Weapons
+5, Fists, Endurance
+4, Athletics, Might
+3 Conviction, Alertness
+2 Survival, Stealth, Discipline
+1 Presense, Scholarship, Lore
-2 breath weapon (fiery breath)
-1 Claws
-1 Wings
-4 Inhuman speed
-2 Supernatural Str
-2 Hulking size
+6 Athletics
+5 Endurance, Weapons
+4, Fists, Alertness
+3 Conviction, Might
+2 Survival, Stealth, Discipline
+1 Presense, Scholarship, Lore
-2 Screech (sonic screech)
-2 Claws
-1 Wings
-4 Inhuman Str
-2 Supernatural speed
-1 Echoes of the beast (sight)
So, maybe I didn't know the rules as well as I do now...
I'm not sure how similar my beast and yours are, but here's what happened to me:
- I rolled poorly and barely managed to hit anyone,
- It takes a full round action to shift, so that became a problem when I found I was not in an ideal form.
- They found his weakness fairly quickly. (within 3 exchanges)
- I was supposed to have some NPC shape-shifters on the side-lines helping the beast with the fight and I forgot about them...it would have made the fight better, especially since the wizard wouldn't have been able to insta-kill them because they were mortal.
I'd say the fight lasted 5 exchanges, max. The the party obliterated the monster. Only one character was seriously hurt but he was the tank and had supernatural recovery.
Oh, also, I allowed mental evocations. That was a mistake.