Cortez is a hard boss, but not the hardest in the game. He was easy for me this time though.

The badges I recommend using is Power Bounce which does a lot of damage to an enemy. Also, Mega Rush is useful too if you're in Peril. These two are really useful for this boss. Using Vivian is what makes this fight a bit easier. It's best if you upgrade your party, but I haven't upgraded any. If you got that Power Rush badge in Lovely Howz of Badges, you attack increases a bit if you're in danger or peril.

HP: 60
Defense: 3
Reward: Sapphire Star
Difficulty: 6.5/10
Recommended Party Member: Vivian

Things you need: None
Things you want: Vivian/Power Bounce/Power Rush/Mega Rush/Honey Shroom/Shroom Roast

HP: 10
FP: 5
BP: 24

Items:

Courage Shell
Tasty Tanic
Earthquake
Honey Shroom
Jelly Ultra
Shroom Roast
Shroom Steak
Fried Egg

Badges Equipped:

Power Bounce
Mega Rush
Power Rush
Quake Hammer
Power Smash
Last Stand