The chance to megacrit is increased by your proficiencies... I can't tell you exactly because I already had my earth proficiency at this level before that update, but people say that the amount of megacrits significantly increases when you get to rank 10 with whatever element you're using
but yeah, basically higher prof = higher chance (also, having a full group increases the chances)
And on that note, I'm at level 5 and just itching to do a guild maze game
who's down with this?