Making him into a levelled list would break lots of scripts -- better to just place an activator into the cells where he fights the PC, with something like this as its local script:
CODE
if ( player->getlevel < 20 )
dagoth_ur00000000->sethealth 600
; [...]
elseif ( player->getlevel < 40 )
dagoth_ur00000000->sethealth 1200
; [...]
else
player->sethealth 0
;for something that'll always be a challenge
endif