Get a feel for the battle. After a while you get the timing down of how long Crota takes to hit his knee. Sometimes you'll be lucky neough to see the bar and know when he's down. Or better yet, you'll have someone shooting at him that knows when to tell you to get there. If you're only getting one hit on Crota, then either you're waiting for Crota to be down before approaching him, or are playing way too cautiously. As others have said, bladedancers using invisibility have an advantage, but it comes down to the person behind the controller more than anything. Watch some videos on YouTube, and look at what they do. When in game, keep an eye on what Crota is being hit with. If you see rockets, get your ass close to him fast, if you see wolfpack rounds, even faster. If you don't see those, then you can play conservative.