Best: if you know where studs are, and can use one of those for at least one of the brackets, that would be ideal. Use a long galvanized or stainless screw for this.
Better: If you use a good plastic anchor with drill bit and screws correctly sized for it, that will be pretty good. Don't use the anchors or screws that came with the shower. Buy good ones. The anchor should be tight in the hole, but still insert fully. The screw should meet firm resistance and eventually bottom out firmly and stop turning. Tile over cement board will provide good hold for a properly installed anchor. Maybe you won't be able to do chin-ups on the shower panel, so, don't do that.
Bad: Half inch holes and toggle bolts. If you are lucky and get this right, you probably could do chin ups on the shower. That's the one benefit. But I think you're likely to have other problems. Drilling half-inch holes in your tile wall in a wet area is undesirable. The toggle bolt won't naturally resist water so you'll have to seal the hole somehow. You'll have to glue the back of the shower panel to the tile otherwise the toggle will allow some movement laterally. If pipes or blocking or anything else inside the wall prevents the toggle bolt from being inserted and seating properly on the back of the wall, you'll have half inch holes in your tile wall to be patched. That's a PIA in any wall, but inside a shower you should really avoid it.