1. Choose the Kensai class. You won't need any guns and the 6 lives will give you plenty room for error.
2. Find a location on the map that looks like this:
3. Your sword has a wide angle of attack and will cover the whole area that is not covered by walls.
4. Sit there and deflect all the shots or kill enemies if they get too close.
(This is a good time to observe AI behaviour, how it moves and when it attacks).
5. Survive 7 times and get the achievement.