they can i just tested this
a warrior and paladin have higher leadership at start and thus have more units in their stacks which easily compensates the mana cost
a mage needs like 35/40 mana(WITHOUT concentration) and maybe 1-2 scrolls of last hero but that shouldn't be too hard to find..
however a warrior can do this with 30/35 mana with 1 scroll
keep in mind that a warrior got way more units
and thus would do more damage to a turtle
with my mage i did like 200 damage a turn(rougly based)
around 40~80 from archimage
20~50 inqi
20~50 bowman
50~96 from thorn hunter
but all units were below 10 except for thorn and bowman i think
bowman 14
thorn hunters 102
(i picked up all leaderships flags before going into the turtle island :>