BTB Leader Assassination questions.

Hi guys our Silent Threat Carnival player has acquired the 35 CP needed for the Assassination achievement and we're all wondering who he's gonna pick and when!

So a few questions regarding some scenarios that have come up.
1. Do you have to use it as soon as you unlock it?
2. If not when can you use it?
3. Can it be used in the middle of a battle? Say against someone using the artefacts making them drop them there and then?

Thanks in advance guys!

I believe that it should be used as soon as the achievement is unlocked. All other achievements come into effect immediately so I believe that it should be the same with this achievement.