I'm writing this so everyone can go figure out for themselves what type is good for a guardian in the future.
Beginner Method
1) Whenever you catch any guardian, open up the Catalog from your Hunt or Profile button and search for the guardian you've caught.
2) Tap the Guardian you've caught and you will open up the Profile of the Guardian with Cool stats.
3) Compare all the stats and check which stat is the highest. Refer to this from Guardian Cross Guide in order to determine which type boost the highest stat even more.
4) Most of the time, that will be the best type after Ace for the guardian. To learn more, use the advanced method below.
Advanced Method
1) Follow all the steps above but in addition, JP Wiki or Guardian Cross Guide or my blog to find the skill set for the guardian you just caught.
2) Look out for any buffs (Haste, Cunning, Might... etc) Those raise the stats even higher, so factor in those buffs to determine the best type. For Dead Moon Desert guardians, the new skill 'Self-destruct' calculates damage from MP so Sexy type may be good for guardians with that skill too.
iTunes App Store link: Guardian Cross - SQUARE ENIX
