Which merc: Depends on your character. For a melee/Missile character I prefer a nightmare offensive merc (might aura), else a nightmare defensive one (holy freeze).
Which polearm: Depends on the rest of the equipment of your merc. Either a cryptic axe (damage) or a thresher (speed). The problem with these two weapons is that Larzuk will give them more than 4 sockets which only leaves the cube recipe, with a 1 in 6 chance to produce a 4 socket one, apart from finding one with the correct number of sockets. If you get impatient with the polearm, get an eth CV which will be significantly easier to obtain. Larzuk will add 4 sockets, but that weapon has a str req of 200 which might be too much for a merc with a rather low level.
Also consider an Act 2 normal combat merc, Prayer aura. You'll have twice the life regen of a single Prayer aura. You'll have the normal Prayer aura active, and also Prayer's synergy to Meditation active too, resulting in double the life regen.
Despite what you might think, the life regen IS actually noticable. Very noticable. Noticable enough to make you stop using healing potions for good. Almost.