Updated.
A wastrel can be acquired as a familiar by a xth-level arcane spellcaster of neutral evil alignment with the Improved Familiar feat.
7th level like imp and quasit?
Seems right. For tactics, they make hit-and-run attacks to establish a life-link and then stay as close as possible without risking attack.
I think an Imp/Quasit's a lot more useful overall, what with their having SLAs, hands and human-level intelligence.
I'd prefer 5th level for a Wastrel.
The Life Leeching is of limited effectiveness when there's just one Wastrel, but their ability to locate a creature they're blood-linked to has its advantages...
Can we compromise on 6th CL for Imp Familiar?
Updated. Are we done here?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.