I have done around 100 matches of Blitz and only completed 3 of the 9 BGs in under 5 mins which are; Eots / Deepwind & Gilneas and the reasons are stupid: Get the most godlike team possible, your enemy team has deserters after losing the first fight, you have an insane MW monk player that flies across the map fast and managed to prevent the enemy team from capping the first base in Eots and took it for himself by knocking the guy off the map.
To do the achievement you need to be lucky enough to get the BG you want while your team is absolutely stacked which isn’t easy when facing geared ppl with identical skills. This becomes more apparent the higher you climb in rating.
But there’s one specific BG that’s currently impossible even if you get lucky and that’s Silvershard Mines. The game takes around 6-7 mins to complete if you manage to capture every cart.