Each time an ADEPTUS ASTARTES model from your army makes an attack, if that attack targets a CHARACTER, MONSTER or VEHICLE unit or if your Saga is completed (see below), that attack has the ability.
Saga of the Beastslayer
At the start of the first battle round, your Beastslayer tally is 0, and you determine your Beastslayer target by halving the number of units from your opponent’s army (including those embarked within TRANSPORTS) that have one or more of the following keywords (rounding up): CHARACTER, MONSTER, VEHICLE.
Each time an ADEPTUS ASTARTES unit from your army shoots or fights, after all of those attacks have been resolved, add 1 to your Beastslayer tally for each enemy unit with one or more of the following keywords destroyed by those attacks: CHARACTER, MONSTER, VEHICLE.
Once your Beastslayer tally is equal to or greater than your Beastslayer target, your Saga is completed.