your doing it wrong then
you have to do it this way
use haste of firefly and move it next to the turtle*(so the turtle will attack firefly)
(turtle acts slow so cast last hero on the firefly)
if you now right click on the firefly you see that is has 3 turns of last hero!
now when the turtle attacks u decrease it by 1 so u have 2 turns left
when u press defend after the turtle attacked another decrease will happen
so you will only have 1 last hero left
if you would press defend there is no more last hero and if the turtle would attack then your firefly is wated
so be sure to WAIT first with the firefly before pressing defend to maximise efficiency
so first turn
haste firefly(his special ability)
press wait cast last hero on the firefly
let all ranged units shoot
keep the firefly on wait
let all shoot(if you have brought a thorn hunter with you it's even slower than turtle) u can cast last hero again on the firefly and defend
then get back to 1
and keep checking how much last hero firefly has left
:>