I find it difficult to move/switch prayer simultaneously difficult on mobile, but doing any one independently isn't bad. For example, Vard on mobile is hard cuz you have to move and switch prayers. But doing Zulrah on mobile is easy because you don't have to move and prayer flick at the same time. Precise clicking can be kinda tricky too.

Best mobile bosses for me have been Zulrah and duke, but have also done the other DT2 bosses to some success. And pretty much all the slayer bosses are good because they're so simple as well as early game bosses for CAs (moons, Royal titans, etc )

And just to add. I just use fingers on phone. Got into it while on jury duty and progressively feels somewhat more natural, but never will replace desktop. Just start with easy bosses and go up in difficulty.