Yiazmat (final hunt) in Final Fantasy XII, It took me 18 hours 20 mins (to beat him alone) but the last few hours when he is in critical at double level (Level 150 ish I believe) taking only 2/3rds damage and killing a (lev 99 with ultimate equipment) party member almost every hit is just epic, such an intense fight.
Plus add the hardest monster from any given monster hunter game in here, their toughest is always a blast regardless of the monster hunter game/version. But that basically IS why anyone plays the game 
Speaking of which I need to buy a Japanese 3ds so I can get M.H 3G....
PLAYSTATION NATION LADY OF JRPGS

Favourite Games of 2013 1.Tomb Raider(PS3) 2.Atelier Ayesha(PS3) 3.Virtues Last Reward (Vita)







