This is based exclusively on the TNG Tech Manual backstage speculation on what settings the TNG Type 1, 2 and 3 phasers have. It is only according to that book that Type 1 has Level 8 as the highest setting, this being the first one capable of making matter go away. Only one choice available for Type 1 users in removing matter; that, plus eight more, for Type 2 or Type 3 users.
Now, Type 1 has been seen removing enemy soldiers out of existence (e.g. "Hide and Q"), as well as blasting holes in doors or other non-humanoid objects (e.g. "Too Short a Season"). It's demonstrably a diverse
make-go-away tool. All with that one make-go-away setting! Even if Yuta were made of exotic materials, this wouldn't protect her from sustained Level 8 fire, which can thus be considered a "mercy killing" setting, something that is absolutely certain to work on her - eventually.
Sure, we have seen certain creatures or people resist phaser fire before. But none of those instances (Rogar in "The Hunted", MacDuff in "Conundrum" etc.) featured explicit use of Level 8 (or higher), the way "The Vengeance Factor" did.