Both online achievements can be earned in one match (3 if someone has the viral achievement. Can be boosted with 2 & will take about 3mins per player.
Go to Multiplayer - Xbox Live Match - choose your character - Create Match
Ensure you select Ranked Match & go Create Match. The online is very dead so shouldn't need to specify the number of slots but you can if you need to.
You can select any map under Mission.
Under Settings, ensure you change the ZGU's to Win to 100 (max) as you can collect 60-70 ZGU's before killing the 25 zombies necessary.
All other settings can remain the same.
Invite your buddy in & start the game once in the lobby.
The player getting the achievement needs to kill 25 zombies using only their Ranged Weapon
while others do nothing. The achievement will unlock in-game as soon as the 25th zombie is killed. Don't worry about dying as you respawn after 10secs.
I preferred to use Amy as she has a fast-firing ranged weapon.