They’re really hard. Partying up with a crew can help a lot. Especially if you have a collector who can get all the motes you make. My friends who have completed the reckoner triumph swear by stormcaller warlock with crown of tempest and a bunch of intellect boosts. Some of them are hunter and Titan mains and they still say this is the best way. They also used risk runner on fallen. Ruinous effigy could work well too... Just keep trying, you’ll get it!!

It's not a simple medal to grind. Your best bets are bottom tree Dawnblade or Poledancer + Raiden Flux. Some players also suggest going ham with Tether + Telesto. It'll go by faster if you party up with a group of friends/clanmates/LFG'ers who will agree to leave waves of enemies for you to kill rather than chancing it with blueberries.