It's not so much that you have to or can't... it's more like the attention span of the average mobile player is they want to do something snappy snappy and then walk away from it. Admittedly if it was a full game that took 10+ minutes per match with multiple units I probably wouldn't actually play the game as I already have FFBE or actual console that consumes most of my time. Having snappy battles is appealing since I don't want to invest more than a minute or two per match.
It's definitely not for everyone but I guess if I wanted a full on Fire Emblem experience with a good story (cause this has a laughably bad one) I'd probably just... buy a fire emblem game and skip the micro-transactions.
Mages aren't a problem for me so long as I don't let the elemental weakness of my own units get hit by them.
AKA Blue Mage on field, I keep my red units out of their reach and send in my green units to kill them as they will do little to no damage.
Biggest problem for me are red units of any kind because I have no strong answer to that. I've been sending in my own red units to combat them with a backup healer. (edit: as soon as I get 2,000 feathers for my M!Robin I'm going to get that dude to 4*, I bet even at 4* he can be a monster)