I've been running hard crota swordbearer all day killing crota in two swords, and I'm pretty sure now that the amount of time you have before he gets back up depends on how fast you take him down. It's a lot harder to get in all the hits when there is a team of 31s, even if I hit him immediately when the shield is down. With a team of 32 gjallahorns I can get in 5 hits with RT, whereas in a team of 31s I sometimes don't even get the 4 hits in. Tell me if you have had similar experiences.