If a glasc mixologist dies in the beggining phase, can the unit generated by its deathknell hold that battlefield and therefore score to 8?

Yes — provided the death happens before your Beginning Phase's Scoring Step.

The Beginning Phase is a Beginning Step followed by a Scoring Step that Holds every battlefield you control (Rule 315.2.b, 469.2). If Glasc Mixologist dies during that Beginning Step — say, to a start-of-phase trigger or a spell played in an Open state before the step resolves — its Deathknell goes on the chain as a Pending Item before it hits the trash, and because an item is on the chain you can't lose control of the battlefield in the meantime (same reasoning as FAQ #9149). You resolve the Deathknell, play the unit to that battlefield you control, and it's still there when the Scoring Step runs — so you Hold and gain the point.

That point can absolutely be the eighth: the Final Point restriction in Rule 471.1.b applies only to points gained through Conquer, and points from other sources aren't beholden to it.

The timing caveat is real, though. If Glasc dies after your Scoring Step has already passed (Channel or Draw Phase), you get no point that turn — the Hold window is gone and you can't score that battlefield again this turn (Rule 470).