Gaara's sand is very fast now,onoki dosen usually move when he is forming jinton,also he can use it only twice befor running out. Jinton takes a while to form,gaara could quickly attack him with his sand relentlessly to keep him occupied. Also it's just a thought,but if onoki fires a jinton at gaara,and gaara hits the cone with sand,it will make a premature setoff right. Gaara can also fly as fast as onoki,he even said he would have reached the island faster than onoki,he coyld move about and attack onoki continously while grinding up the roks to make more sand to use.