Its not hard if you have remotes on all of them then they can do it automatically and you don't need to control them. Can even leave them afk while you go to school, work or sleep and come back to them when they need food or what ever. If you are just training and not bursting then you can get your water Soul dodge skill. make sure they have recovery and revive. and your team should be fine because the water will heal them. and soul dodge will replenish its sp so you will never need food. But keep some sp food on just in case there is a glitch and soul dodge fails to work.)

Its also not hard to lvl your team to 20. You can get to lvl 40 in a week. and if you have a training voucher and go to training island also possible to get to 40 in a day or 2. Especially if you have potions.
(you can get;
a 2.5x from Houyi in welling village, catch him a bunny
a 4.5x pot from cupid in holy village, do a valentines pop quiz
a 1.5x from the lady behind the mayors house in holy, she wants you to go to cook's ship to talk to her husband elvis
a 1.5 from cooks ship, he wants you to return the badge. choose the good option to help the sailor.
a 2x from elin quest, get elin.
a 2x from sam's quest, in the basement click on the 4 different gold piles.
and a 2x from the secret room in the mayors house in holy village, room is behind the butler on the first floor click on the burned out touch lamp on the wall to enter. first chest gives you pot there is a second chest in a back room that gives you 5 ship vouchers... or vice versa... been a while since I last did that quest)

All those should be able to help you lvl a little faster (but best to ONLY use pots on double exp days, because you get more benefit from it.) Also best to use Training Island Vouchers on double exp days as well.

As for GS you can get it anytime even at lvl 1. Some people just rather you be 10 or higher because you might need CON so you don't spawn. But with a good team you can get it at lvl 1.