Dustin gets fully hit by the spray (14 Ref save), taking 8 points of acid damage.
Round 478
When River pops his head up, Dustin rakes him with a claw (rolled 24) for 11 points of damage.
River responds by breathing acid, which Dustin takes fully in the face (15 Ref save), taking 11 points of acid damage, along with the 4 points of acid damage from the previous spray. Dustin has 24 HP remaining. River can breathe next round.
Round 479
Dustin hits River with both hooks (worst roll 22) for 18 points of damage. Dustin grabs River and pulls him out of the ground (29 vs. 29 grapple; Dustin has higher modifier), biting River for an additional 23 points of damage. River is unconscious, and defeated.
Dustin is at 70N20E, and has 17 HP remaining after taking additional acid damage.
Next in round 479 for Neutral Evil is a chimera skeleton (Monster Manual).