We had a somewhat similar situation last session. The heroes set up
an ambush. The Yinkin ambusher had plenty of abilities to rack up
augmentations. IIRC, I let the Yinkin use his Cloak of Darkness item
as an augmentation, and I've relaxed the rule of allowing only one
augment. The other hero (an Odayla) tried to make a trap at the
ambuscade, to further hinder the ambushee: another augment to the
ambusher's ability (to no effect, it transpired). Relaxing that rule
allows the Narrator to include all the preparations the
assasin/ambusher wishes to use.
But the most important thing in the ambush was that the ambusher
initiated the contest, and so limited the choice of abilities the
ambushee (a draugr) could use. I ruled that the draugr's large Close
Combat (Wrestle) was inappropriate; what's needed is a Perceptive or
Quick Reactions ability. So the poor draugr had to defend the
initial attack with a default ability of 6, with 6 APs. The Yinkin
bid high, with the result of an initial Hurt. A Coup de Grace in the
next round completed the ambush.