Don't know if this has been suggested before but instead of effectively giving the player a loss when they get disconnected during a solo fight, how about either 1) give them a chance to reconnect like in co-op and/or 2) have the AI take over and finish out the fight. This way, if the player was losing already he will still likely lose and thus will gain no advantage by purposefully disconnecting. Perhaps more importantly, if a player is winning and gets disconnected through no fault of their own, they will still have a chance to win the fight.