From Archon Arcana
If the word “before” is used in an ability (for example, “Before Reap:” or“Before Fight:”), that ability resolves before resolving the game effect of the reap or fight (but after the card exhausts, if exhausting is required to use the card).
—Official rules v1.7 December 2020
What happens if I use a creature with both Assault and a Before Fight: ability?
If a creature has multiple "Before Fight:" abilities (including Assault), and that creature is used to Fight, all abilities in the Before Fight window must resolve before the fight proceeds, even if the attacked creature is destroyed by Before Fight abilities.
Source: Crucible Cast, Episode 4