I used a very CHEAP trick to kill the turtle..
find some eggs or w/e(used items) which summon creatures
have 2-3 stacks of ranged units
move your summoed troop to the turtle*(he will cast slow first) speed your unit to nuliffy this
keep him close to the turtle so he is the only one that is being attacked:p
keep casting LAST HERO on this unit(since it will die anyway)
and let the archers keep shooting the mofo ^^
i'm sure there are easier ways but i found this one very funny(doing the last hero on my peasants that i got from my tutorial)
the turtle dropped in no time ^^