1) look for groups in PvP to take out, Control is great for it. Make sure you activate it AWAY from enemies, you can be killed instantly if you don't.
2) players have gotten much better at defending it, I used to take out 3+ players every time, but now players who are good with a shotgun can kill you pretty easily. Sometimes Hunters will use Blink to get away from you. Some players (Titans) will just super you before you can kill them.
3) blade dancer in PvE is pretty much useless
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On the PvE side Bladedancer with crouch to invis + shadowjack to extend invis you can and will be able to revive your team pretty regularly. Saved our butts on this week's nightfall.