Out of the four current races to fight, which are the most tedious to fight? For me, it's the Vex mainly because of the Hobgoblins and their dang shields, and their headshot cores are harder to hit than other races.
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Cabal with arc burn (psions can shoot through a titan bubble a.k.a. you're never safe) and hive with arc burn (boomers wizards, and thralls oh my!)