That is what Desecrate is for!
Desecrate
Evocation [Evil]
Level: Clr 2, Evil 2 ...
...An undead creature created within or summoned into such an area gains +1 hit points per HD...
If the desecrated area contains an altar, shrine, or other permanent fixture dedicated to your deity or aligned higher power, the modifiers given above are doubled... +2 hit points per HD for undead in the area).
Given that a an outsider has a d8 HD and thus gets about 2 HP per die more from a d12, it is still sort of like having an 18 Con once the Dececrate HP kick in. Less wretched than one would think. Improved toughness should be on the 'feats to take' list for the lich fiend none the less.