The gambit one will surprise you. I think the only times I went into Gambit with the intent of completing one of the Triumphs was the Protect the Runner and maybe the match where you had to win using all Gambit weapons. The rest just happened over time with playing. You may need to glance at them here and there to to adjust your playstyle but its not bad.
What took the longest was Light vs. Light, by far.
What took the longest was Light vs. Light, by far.