Earth main seems more useful alongside premium event ninjas like Naruto [Sage of Six Paths], Kakuzu [Earth Grudge], Hiruzen Sarutobi [Edo Tensei] etc.

Well, like the above person said, most meta ninjas currently fits well in an earth team. So basically, you'd need those ninjas for him to shine. F2P-wise, Lee full bt or Shira 4* brings out the best in earth. P2W wise, you'd want one or all of the following: 6PN, Edo Deidara, Kakuzu EGF, Sutachi, Madara Founder, Shisui, Edo Hiruzen, Edo Itachi, etc. Those are just some meta ninjas that works well together and works well with earth main late game.

one of the cheap ones would be, lee skillbreak, yamato (for barrier) guy (for buff and regen) and earth main with earthbuff and prob double attack mystery since it clears control (chaos/paralyze/acupuncture etc)

As soon as you get 70k+ coupons worth of ninjas

yes, is as they told you.

wind and earth main strictly require good ninjas to work fine.

water and fire work good also with relatively bad ninjas.

fire is a half way.