That's why, when you get to the last sword (after the ogres) take 2 guardians to kill Crota. The main guy hits Crota, he engrages, if he's killed the 2nd guy (who's hiding on the platforms where swordbearer jumps from) then jumps up while the remaining guardians down dickface for the last time. Boom, boom. Done. It works every time with this strategy.