IS#5 vs IS#6 first boss stats to compare the difficulty.
>Fremont
330k effective HP (2 phase), 3300 atk, 1500 def, 80 res

>Yi
1 mil effective HP (2 phase), 4300 atk, 1800 def, 60 res

Yi is somewhat near Boat Fremont, statwise. The boat versions of bosses were basically HG trying to fix the low stats joke IS#5; this time they have it right from the start.