Legion Tokens stack up to 25k.
Choose your method:
Bright Paragon - Fastest
If you happen to own this pet, complete its quests by defeating enemies at /brightfortress. The best room to do this in is the one with Commander Yulgar and 2 BrightScythe Reavers. This is the best method if you own it but unfortunately the Bright Paragon pet isn't available anymore.
Shogun Paragon - Fast
This is the best method for most players. You need the Shogun Paragon pet for it, which you can buy during Dage's Birthday event in March-April each year for 2000 LTs from /futurewardage. Farm its "Time for Some Spring Cleaning" quest by defeating Fotia Spirits and Elementals at /fotia. The drops stack to 50.
Dreadrock - Slow
Purchase the Undead Champion armour from Dage's Legion shop at /underworld. Use it to accept the "Undead Champion Recruitment" quest from the quest board on the 2nd screen at /dreadrock. Complete it by defeating enemies left of the quest board.
Legion Arena - Slowest
Complete the "First Class Entertainment" quest from /legionarena by defeating the Legion Fiend Rider (4% drop rate). This can be more efficient than Method 3 but that would require a group with Chrono classes and supports.
Alternatively, you can complete the Arcane Paragon's quest, but you'll need to be an endgame player with a Chrono class to make this work. Other Paragon pets include Festive, UW3017, Mounted, Dreadnaught, Holiday, Thanatos, Ascended, Regular, Hardcore, and Infernal Caladbolg. If you have them, use them until you can get Shogun (excluding Hardcore as it's not viable to farm). Generally, only the Bright Paragon is worth doing over Shogun.