1) All three
Actually, with good tactics, a poster here did it with a Mage in 7 days. Another player did it with a Warrior in 10 days. As a Paladin I did it in 12 days. So regardless of this info, all three classes can do it.
2) Generally, that's what this means. You won't have that much magic runes to delve that deep into the magic tree, and skills like Transmutation and Summoning are a must to minime the casualties - Paladins + Phantom spell + Summoning lvl 3 = More phantomed paladins = More resurrected units.