Ok I just calculate the duration of the polimorf spell and I what to know if my calculations are correct and if I'm missing something
Talent: gifted adept (shapechange), magical lineage (shapechange), enduring spellcraft (azlanti)
Item: orange ion stone
Arcanist: potent magic, spell thinker
Feat: extend spell (if not playing brown fur archetype) spell specialization (shapechange)
Mythic ability archimage: arcane endurance, channel power, shapechange mastery
Mythic feat; mythic paragon
Calculations; casterlv 20+1(gifted adept)+1 endurance spellcraft+1 ion stone
+2 potent magic+2 spell specialization+4 arcane endurance+6 shapes shifting mastery=37x2 extend spell X2 channel power x1.5 from spell thinker=222x10 min (the duration of shapechange= 2220/60 minutes (to pass to hour) 37 hour
Other polimorf spell will last for 3,4 hour.
Str buff whit brown fur archetype lv20 (+4) str+ improve Eldritch heritage abissal (strength of the abiss) (+6) + mythic spell augment +(4) so u get a form of the dragon who str +24 con+12 and a base attack +36
The way PF stacks multipliers, 37 x2 x2 x1.5 = 37 x3.5 = 129.5
i.e. 21 hours 35 minutes duration on that shapechange.
Form of the dragon accessed via shapechange wouldn't be the mythic version I think. If you learned the mythic version of form of the dragon III, cast it separately from shapechange and stacked up those bonuses on it you'd get Str +10 size, Con +8 size from the spell, +4 increase to Str or Con (not both) from brown fur for 2 arcane reservoir points, IEH (Abyssal) Str +6 inherent, mythic spell augment +4 untyped to both for 2 mythic power. Assuming that you put the brown fur bonus to Str you could have Str +24, Con +12.
Shapeshifting mastery giving you BAB=caster level is broken even as mythic stuff goes. Like I said I don't think you could use normal shapechange to access mythic form of the dragon, but even so that's a lot of BAB you're looking at.
BTW those misspellings hurt...
Sry English is not my mother language it Spanish I enter in this forums to practice
The 1,5 apply 1 round later because I can't use potent magic and spell tinker in the same round
Shapeshifting mastery giving you BAB=caster level is broken even as mythic stuff goes. Like I said I don't think you could use normal shapechange to access mythic form of the dragon, but even so that's a lot of BAB you're looking at.
BTW those misspellings hurt...
Even if I did use form of the dragon 3 I only remove 7 lv from arcane endurance, gifted adept and spell specialization and still have 29 bab