I interpreted his response as, essentially, "create a second object to manage the boss sprite".
But regarding the SetDestRect, suppose your boss image is 64x48. You'll want to manually set the dest rect so you can visually move the sprite, as such:
ObjSprite2D_SetDestRect(obj, -32, -24, 32, 24);
That will center it. To make it move up and down, introduce a sin variable that will add onto the y values.
let m = 0;
let mv = 0;
(loop){
ObjSprite2D_SetDestRect(obj, -32, -24+m, 32, 24+m);
m = sin(mv)*5;
mv+=9;
}