Okay, so, when battling groups of different difficulties, how do you guys do it?
f.x two 2-stars and 1 3-stars, do you take out the 2-stars first or the 3-star?

I'm split between those, since, taking out the weaker first means less enemies to hit you, but you still have the stronger one to hit you for a longer time. Reversing the two, you take out the strongest one first, but will still have several enemies hitting you..

So, please share your strategies ^^

I think I really differ depending of the group of Ennemies
I have in my face... P=

Let's say I'm in a fight with 1,2 or 3-stars, I got for the
strongest first, hit only one, then when I'm over, I'm
taking the next strong one down~ XP
If it's in a battle like with Tyrielle from the Valentine
Raid where there was 2 4-stars and Tyrielle(5 stars)
I'll focus on one 4-star first, like do 3 Finishers, hit it
until it fall, having my mana almost full or full, I now
take care of Tyrielle and leave the last 4-star for the
end~ P=

Also, I sometime switch a bit my strategy if there's a
pet in the battle I WANT to catch~ XD

I take out the boss first, and then the weaker ones. I normally save up mana along the way to the boss first.

Regular battle I just aim for the strongest one since I have the fire ball skill, and hit two in one turn.

Mm, I like to take out the 3x star before the 1x and 2x stars. Or if it's a battle that doesn't matter, I usually split my team to take out the 1s or 2s with some siphon/lacerate effects.

Very happy with your inputs! my battle team is made up of earth-tank, a light, and an ice. Is it recommendable to have back-ups with dark and fire, to?

Well I put dark on my tank, but doesn't really do much as far as sucking up HP would go. Still recommendable on a pure-int build.

As for fire, people love it for AoE madness, BUT ONLY IF they're on pure int for max damage. People use 3x teams of pure int fire pets for clearing rooms and waves of raids.

tl;dr - yes, but with pure int build.