Here is the full list of all 6 Butsbal achievements. It takes around 0-1 hour to unlock all of the achievements on Windows.
Capture the flag 10 times in a four or more player match
Win 10 or more rounds in a four or more player Last Man Standing match
Kill 5 opponents without dying in a four or more player match
Win a four or more player match without suicide.
Kill 3 opponents without dying in a four or more player match
Capture the flag 5 times in a four or more player match