To get instant victory, you need to use the "Boost" button to charge against the enemy when it's near and in your direction.
There is also a condition where your character level needs to be higher than the enemy (more than 4 level above I think), otherwise you can see the arrow color notification. If the arrow showing the incoming enemy is blue then is should be possible.
The tips to achieve this quickly, is to let the ennemy coming to you and when in sight click boost button (L3 on playstation) to charge against the ennemy.
You may need to try several times to find the right timing.