Requirements:
Be a mage or a druid.
Engineer with Goblin Glider
Bottled Tornado
1. Use your Bottled Tornado + Goblin Glider combo here
https://i.imgur.com/iok4B.jpg
2. Use Blink or Displacer Beast here to get over the invisible wall. (Puntable Marmot or Darkmoon Tiger might work, haven't tested myself tho.)
https://i.imgur.com/C8ntg.jpg
3. Use the combo again.
https://i.imgur.com/Q3Khz.jpg
4. Walk a little to the right to this spot. You need to get past 200% speed. Rocket boots works just fine. Jump from this spot
https://i.imgur.com/2j0zf.jpg
to this spot.
https://i.imgur.com/IKRCj.jpg
5. Once again, use the combo on this spot.
https://i.imgur.com/fRVIu.jpg
6. Then just keep walking left.
https://i.imgur.com/qBVf5.jpg
If you glide a long way away from the map, you will fall down and die and respawn at Westfall graveyard.
https://i.imgur.com/j3q9S.jpg
https://i.imgur.com/xiKtk.jpg
Video of us doing the bug.
How to: Get out of Map with the Flag in Warsong Gulch - YouTube
Nothing prevents you from taking the flag with you.