She's great as a lightning rod, aka you place her AFTER your other units are placed, then she will absorb all the ranged attacks, as well as letting units pass and only blocking units when necessary. her high MR and def allows her to protect your healers and other dmg-dealing units.
watch: https://www.youtube.com/watch?v=KIajXd_oS1E
starting at 2:00 or so, silver angel niel is used to protect healer iris in the previous star rush event G-tier map. if you dont place an angel there, the flying mobs will kill your healer. I've tried.