On Fire?

Seven arrows of health degeneration.....is this like the enemy loses health or something?

yup, you should see his hp bar drop very fast with 7 arrows

is this on fire thing like the phantom skill that the mesmer has in that fashion but faster, and is it trully worth 1-3 sec of seven arrows? or should i stick with spells of 5 degeneration of health for 10 sec

Fire is very useful because it is on short timer...doesnt give enemies chance to remove effect unlike something that lasts 10 secs or so....it may last longer but the player has much longer to remove poison, phantasm, or faintheartedness ect...by the time they cast a remove spell fire has faded and they wasted energy trying to remove it(if they are even able to act that fast)

about how much health does it take away from enemy?

It reduces life by appx 2x the ->. Yeah, I realize that doesnt make much sense, for fire damage its basically 14 damage per second I believe. The nice thing about health degen is it bypasses armor and resistances.