When trying to pick a balanced team to level, you look primarily at the element, not melee/ranged/magic. Elnora is not in competition with Chharodi, nor is Meryl in competition with Trisha or Emily in competition with Naladia. Chharodi is in direct competition with Trisha, however, as they're both earth. Similarly, you choose between Elnora and Emily.
Two star characters can be purchased with tokens in the shop. Three star can't. When they're up in the shop, for a two star girl, you can get about 3 shards/day for trial shop, 4/day for competition shop, or 12/day for league shop, and you can do that for a solid month. That's in addition to showing up in the farmable events with higher drop rates. And you can also accumulate tokens without spending them in months that you're not interested in the girls that a particular shard offers, so that you can get 12/day or so for a while after they rotate. It will still take a while to get a two star girl to seven stars, but it's at least doable. Three star girls will take, if not forever, then awfully close to it if you're not a whale.
I'm not sure what level of PVP you're fighting at, but in the top tier, that's wildly false. For reference, I'm currently #4 on my server in squad power and #18 in arena--in spite of never spending diamonds on arena and rarely spending gold. I often draw, and sometimes challenge and beat, players ranked in the top ten of arena on my server.
My arena team consists of Elnora, Naladia, and Chharodi. I don't have a strong wind character (my 190k Zephyr isn't good enough to be worth using, though I may give her a shot once she reaches seven stars in a month or so), so strong earth opponents are the ones most troublesome to me.
Trisha and Claire are both easy, as either my Elnora or Chharodi will handily destroy them if the power rating is at all close. Ceilim and Dora are somewhat better, but lightly used, as it hasn't yet been practical to get them to seven stars. The whales seem fond of Eremia, but I haven't had a chance to face her on a team that wasn't overwhelmingly powerful, so I'm not sure how good she is.
An opposing Chharodi is the character I most fear, and currently the only character in the game where a 330k power character inside of an 800k power team will probably scare me off. That doesn't mean that Chharodi is the strongest character, period, but only the strongest earth character that was in the game at launch. Strong girls of other elements can be countered by my team by whichever character has an elemental advantage unless the power rating difference is overwhelming.
If you're running around healing in Tower of Trials, you're doing it wrong unless that's a last resort to try to bail you out after you messed up and let one of your main characters die. Healing is completely useless there, as you heal from winning a match. You save your SP for heavy damage skills so that you basically one-shot your opponent with a damage skill.
Claire might plausibly be useful for healing in level 50 raids if you've got teammates who can kill the raid boss, but that's the only thing she's much good for.
What does melee have to do with Khalis? If you want to counter Khalis, you throw a fire character at her. My Elnora handily destroys any Khalis I've ever fought. If I need Elnora elsewhere in arena, I sometimes have Naladia fight Khalis, and they basically fight to a draw until she gets help if I have only a modest power advantage.
Of course Trisha is going to die to Khalis. Wind characters beat earth characters, on top of Trisha being terrible.
Isabel is fairly tanky, and often the last opponent to die in arena. But while she can heal, she doesn't dish out much damage, so her being tanky just means that she's the last opponent alive before getting overwhelmed 3 on 1. My Chharodi handily beats her if the power rating is at all close, and Naladia can usually beat her if the power ratings are even. Isabel would be a pretty strong water character if she were easy to star up, but she isn't, and if you're giving up 100k power as compared to another character whom you could have gotten to seven stars, that's going to make her useless.