For this you need to loot the elixir of ancient knowledge from Krol the Blade in the Dread wastes, and full heirlooms. The potion gives an alt a 300% xp boost for one hour. This is BOA and can be sent cross-faction like heirlooms.
Level up a horde alt to within a few points of lvll 15. Get your experience locked by the trainer in the Valley of Honour at Ogrimmar. Complete, but do not turn-in, the following quests:
Horde Runaway Shredder!
Start Labor Captain Grabbit
End Automatic
Level 11
Category Azshara
Experience 875
Previous Report to the Labor Captain
Next The Captain's Logs
Investigate the wreckage
Start Gazrog
End Automatic
Level 15 (Requires 13)
Category Northern Barrens
Experience 800
Previous The Purloined Payroll
Next To Track a Thief
Love it or Limpet
Start Gazlowe
End Automatic
Level 16 (Requires 14)
Category Northern Barrens
Experience 1150
Rewards 16s
Previous Club Foote
Next Ammo Kerblammo
Memories of the dead
Start Malynea Skyreaver
End Automatic
Level 12
Category Azshara
Experience 900
Previous A Thousand Stories in the Sand
Next Mystery of the Sarcen Stone
Transdimensional Warfare: Chapter II
Start Ambermill Dimensional Portal
End Automatic
Level 18 (Requires 14)
Category Silverpine Forest
Experience 1700
Reputation +350 Undercity
Previous Transdimensional Warfare: Chapter I
Next Transdimensional Warfare: Chapter III
All of these quests have a remote turn-in, they can be completed anywhere including BG's.
(Note that the last two quests, especially Transdimensional Warfare, are a way down the quest chain. So, you might just want to do the first three which are very quick and settle for getting a lvl16 in the 10-14 bracket. )
Turn your XP back on, Enter a BG, drink your elixir, and complete all the quests. You should now be level 17. Have fun one-shotting everything and being invincible.
If you prefer you can wait until you are low on health and turn in the quest/s then, as going up a level instantly restores you to full health.