Well you have many options for beating this lilligant.
1 hold a lum berry or chesto berry.
2 get a pokemon ex: braviary/emboar/leavany/ etc... to use a powerful (physical because he is useing quiver dance) attack to take it out before it can do anything or if it has already stat boosted or something.
3 get a pokemon like leavanny/scolipede/volcarona that has 4x resistance to grass, since grass is his only attacking move.
also you could get lucky and get a hit cause sleep powder isnt 100% accurate.
hope this helps you take out that flower looking/life sucking/stat crazed/bore-you-to-sleep demon. :)