Being able to go back and replay an area like the Esior's fields would make it easier to level up lower level/weaker pets.

You can replay any level except Esior's Fields, actually, just there will be no storyline there ^^

well that's a problem if you want to raise the level of a weaker pet like a level zero freshly generated, as far as i can tell Trollsblight cavern seems to have an average level of 9 so a freshly generated pet of level 0 has almost no chance unless it has a ton of high grade items or is on a squad but expanding the party is expensive.
So a weak area like the Eisors fields is a far better option for leveling up weaker pets.

@Lunarelupo,

After you unlock more than one slot for a party, you can go back with a high level pet and a level 0 pet in your party at the same time. I actually did this recently, and there were no complications. It works particularly easily since attacks generally go towards the middle party pet.

Not the point I was trying to make.
If they don't want to make Eisor's Fields re-playable then maybe there should just be a Eisor's training grounds for pets under a certain level like level 8 or something.
I find battle the easiest way to determine how a pet's stats should be allocated and I like to do that at an early stage, and putting it on a party honestly defeats the purpose of individual evaluation.
e.g should pet 1 focus on offense or should pet 2.

I remember hearing in general chat a long time ago that when the site gets revamped that we could possibly go through the storyline all over again. I don't know if it's still a possibility or not though. But it definitely has been talked about.

Hopefully not going too off topic. But regarding trying to decide what role to use for a pet on a team I would just look at the base stats on the pet's profile. A pet with higher health or agility would be a more obvious tank to me. A pet higher in strength or intelligence and perhaps a nice helping of dexterity is a pet that I would use for glass cannons. Just depends on the type of team you like to use.

Because I see you're a new player, I also want to mention to try avoiding relics with split damage between physical and magical damage. You'll want relics that have all magical damage for magical damage proficiency trees and all physical damage for physical damage trees. I know though it can be difficult to acquire just the gear you want when you first start.

You can message me if you like if you have battle questions but real life has made it where I've not been able to be on as much lately and I could be slow to reply. There's also guides regarding battling as well.

And if you have unlocked missions, pretty much any low level pet with any armor can beat the level 0 mission and get experience that way. I've never had any pet fail at the level 0 mission in Esior's Haven.