I've been working on trying to get the studs in plane to prevent the need to float. But there's also bows on them.You may be able to pack out the stud work to help get the drywall sections reasonably flat. This can save a lot of messing about with a screed. Also, if you are fixing tiles, unless you are Italian that is, some variation in plane isn't really that noticeable and the tile adhesive can be used to pack out a millimeter or so here and there.
If you are doing a complete wall of any size then I recommend investing in a plastering float, assuming your trowel is what I call a trowel.
PS. Don't forget to fix your first line of tiles just short of two tiles high from the floor!