3 sec is my personal experience. 3 rogue archers 1 mage full crit build. all cunning gear including mage, all masterwork weapon t3 with Bianca t6 aim.
hail of arrow focus with full draw spam, all weapon with dragon killer rune, all members wear crit damage ring, cunning amulets and cunning hat.
drink fully upgraded mighty offense tonic follow by tears of the dead then open combat with focus
pointblank shoot activates full party auto attack with near to instant cool down on full draw and zero cool down on long shot.
full draw till dragon die. no need to enter tac cam.
edit: 1st full draw double spam opening preparation 3 sec not counted because combat not initiated. damage roughly 90k with focus
double long shot as filler 45k follow by double full draw 90k.
roughly damage output in 3 sec is about 225k with the the rest of 75k hp removed by team members.
edit 2: 1 full draw is 20k (extra 100% damage from tonic), focus ability makes 40k. double spamming is 80k+
5 sec kill 13 tries, 4 sec kill 2 tries and 3 sec kill 1 try. it's lucky but achievable.
mage combo kill 3 sec in youbtube. search for it but with pause.