Pretty difficult. The first boss can't be targeted by spells until you killed its tentacles, and will do AoE damage on everyone as long as they are up (you really want Intimidate to work). Tentacles should be Power Break ASAP, and everyone should be fully healed before even reaching that fight, unless you are prepared to S/L a lot if one of your already wounded character takes too much damage.
The good thing is that it is weak to Fire, so you can exploit that weakness.
Sinspawn Gui can absolutly destroy your team if you are not careful and/or unlucky. Your priority is to Magic Break its head, and keep it that way (it will cast Thundara on everyone quite often). You can only reach it with Magic or Ranged attacks, so Wakka with a ranged weapon must be your Magic Breaker. If the head "moves in an odd way", just attack it to cancel its next attack (otherwise, you will suffer).
This boss is immune to paralysis. You will need damage mitigation if you want to succeed. Bring Tyro with Sentinel or Healing grimoire as your friend summon. You must kill the body to kill the boss. Arms will respawn (if you kill those, its body's defense will drop), head won't (it seems to have between 10K and 15K HP, need confirmation).
EDIT: Nevermind:
you can find the enemy's stats here.
If you feel like cheating, just use VIT0.