Summoner is the easiest (and the most boring for most people although I don't agree) class of the game, thanks to its pretty good and safe skills like Alter Ego/Alter Ego Harmony (which makes you the tankiest class of the game alongside Étoile) and Dia Master (which basically means you have infinite HP.)
For the most part the game won't represent much of a challenge if you play Summoner (it still isn't with other classes anyway) until you can play on Ultra Hard difficulty.
In UH, unless you're using Jinga/Wands or Vulcan, almost all the pets will die in 3 or 4 hits, they can handle a few more hits with Alter Ego + Support Roll/Courageous Roll, but they can still die pretty easy if you're not careful, and thus reduces your Pet variety usage. Shame we never got ★15 Pets, I think that could have fixed the problem.
Also, rising a pet to its highest level (130) and increasing all its plus values is rather time consuming and certainly not fun due to how limited the process is.
I would encourage using all the pets there are and stick to the ones you like, that way the class will be less monotonous to play, and also try to experiment with different builds in your candy box, but if you want yo find the most optiomal build (which is for bossing) there are different guides only, and it's certainly essier for us now that we have a way to get pretty much every single sweet of the game, unlike the JP version.