Avoid Dochuki's first two attacks (Bite and Eye Beam or Poison Snot) before 
countering with an attack of your own. When he is stunned, charge towards 
him and press Up. You will crawl into his mouth and see his tonsils, his 
weak point. Attack his tonsils savagely before he spits you out. 

(Note: During the encounter, the far right wall offers security from 
most of Dochuki's attacks.)