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Might be the wrong forum, if so, sorry.

Say I have a squad of 10 Space Marines, all with Bolters. They Fire at a unit consisting of:

1 Wolf Lord with Terminator Armour and a 4++,
A Rune Priest with 3+ Armour save,
7 Grey hunters (3+)
and a Wolf Guard Battle Leader with 3+/3++. The characters stand up front and take all of the fire. How do I allocate wounds and do saves? Do I do one at a time for the closest one until he's dead, and move on to the next one?

Since you have mixed saves and/or characters you have to resolve the wounds one at a time

The rulebook is pretty clear on the matter. From 'Allocate Wounds':

First, allocate a Wound from the Wound pool...

The rest of your question is covered in 'Take Saves & Remove Casualties'.