I've been poorly positioned to deal with the tower events before now. If I'm remembering pull order correctly, then...
When the first one came around I had OSuna, OSilica, Hina Premiere and Silica, and Schoolgirl Philia as usable fighters, which ALO Seven as my only worthwhile healer. I had other 4*s, they just weren't very good (SAO Asuna and Philia, ALO Rain, maybe some others). I'd even neglected to farm enough materials to fully unlock Rechka. I didn't even clear floor 50.
When the second tower came around, I'd added Alice V2, maid Leafa, fox Silica, bride Sinon, a finally-leveled-up Yuna, wind mace Yuuki, and Sakuya to the usable pile, and I'd taken the care to level up some 3* healers, so I did considerably better. I still only managed to clear floor 70. Maybe I could have tried to push further, but by the time I'd gotten there I was exhausted of it, and I didn't think I could get much further with Sakuya as my only water unit.
My bad luck to only draw bow characters from the tower gachas also factored into my failures. There's little point in learning when to parry when the character can't parry.
I feel like the earth tower will be the one I finally clear. Wind is now my strongest element, and I think the free characters we've been getting (Sachi, Bikini Yuna, Korean Kirito, and I'll count Freerito since I didn't farm the materials for him before) should provide decent added fodder if needed.