Does attacking Ochran Assassin ( with +1 counter) kill both Swiftblade Vindicator and Garrison Sergeant with double strike?
In this situation, the Assassin dies in the first strike combat damage step, before it has a chance to deal any damage. Your opponent chooses the order in which the Assassin deals combat damage to your creatures, but that doesn't affect when your creatures deal their combat damage.

Both of your creatures have double strike, so in the first combat damage step they deal a total of 4 damage to the Assassin and the Assassin dies.



If the Garrison Sergeant did not have double strike, this would play out differently. In the first combat damage step, the Vindicator would deal 1 damage to the Assassin. So the Assassin would have 1 damage marked on it so it would still be alive. Then in the second combat damage step, simultaneously, the Vindicator would deal 1 damage to the Assassin, the Sergeant would deal 3 damage to the Assassin, and the Assassin would deal 1 damage each to the Vindicator and the Sergeant. Then all 3 would have taken lethal damage, so they would all die.