As i remember Last man standing achievement used to be different..
I do know (like around 2 yrs ago) person in my team (and we were at same town at that time) received it by being last one alive in our team at end of battle.. yes, other side had plenty of players left!
And when did anyone saw both teams dead at end of battle (real battle!)?!
How high chance is for that to happen.. only 1 person living (from both teams) till end of battle?!
Maybe if friendly alliances arrange it, but that would be cheating, right?!
Well.. i am for setting this achievement to being last person standing on your team and not from both teams, cause really.. that one will never happen in REAL battle!