Still a bit confused when the arena boss's chip states 1-6 attack and a 6 for an alternate attack (or is it an ability). Page 12 states that an alternate attack replaces a basic attack. If you roll a 6, do you have 2 attacks, or a choice between the two?
If the Boss has a 1-6 basic attack and an ability on 6 it will activate both if you roll a 6.
If it has 1-5 base attack and an ability on 6 it will only activate the ability if you roll a 6.
So Dekan the lion will only "Chase down" if you roll a 6 and deal 4 damage to nearest unit only.
Since all Bosses only have 1 in attack range this means that they would need to be adjacent to your unit to be able to hit you with both the base attack and the ability.