I mean you can also just start in "easy mode" which really just lets you start with 4 employees as the free month without pay isn't really that meaningful a change to difficulty. I always go with Cagliostro, Djeeta, Gran and Lyria.Easiest start in Blue Resort is selecting "Djeeta paired with Lyria" in the starting options and then using your free room invite on either so you get both. Djeeta is a good attacker with a good attack buff who eventually learns a very strong multitarget skill that matches her element. You can use her to grind up anyone else you want. Lyria is a good buffer who gets Ultimate meter on buffs and has a very good heal.
Cagliostro and Lyria have a good range of healing skills and buffs and like you said Djeeta and Gran have a really good aoe attack(And if you give them the fighter job they get a skill that maxes their ult gauge so they can spam their ultimate every other turn to deal high single damage too) so they can pretty much get you through most of the dungeons with very little grinding.