50 Heroic Public events is just stupid, do you realise how much time that is? its around 5-10 minutes to complete a heroic public event from (flag spawn to completing) thats around 5 hours (excluding down time between each public event as they're not always up to swap from one after another) so at least another 2-3 hours if you account for that "gap" so 8 hours ez to do this lens.
Seriously change this to normal public events and cut it down to something like 10-20 because no one does public events anymore and the time frame associated with the other lens requirements makes no sense why this is absurdly higher or make it take into account how many you have done previously
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Fallen Glimmer Drill, Cabal Excavation and Spire Integration are your best friends for this triumph. All three are easy to make heroic and are fairly quick to complete. EDZ is great for farming public events because there is usually one happening and another one in the queue to start not long after completing one. Nessus is also a good alternative (however - avoid the Ether Resupply in the Exodus Black area as it's bugged where the Servitor Prime stays immune after destroying first smaller Servitor)