Sector24, good luck in your adventure to complete wo looses...
Some hints from my mage (impossible, 0 looses):
1. Dont count on necromancy talent (if nothing is changed then ressurected units by talent is counted as "killed")
2. Be careful to not overupgrade spells/SR skills (sacrifice, time back etc.). I made such mistake, so in my opinion Sacrifice must be lvl 1, time back - up to lvl 2-3 units... But anyway, it is heavily depending on your gamestyle and Paladin one will differ from Mage one
3. All lvlup upgrades must be aimed toward tactic you will use against dragons (Because when you will get dryads from elves till dragons fights will be "piece of cake").... Just you must know what to do, if there will be 3-4 stacks of them with about 10 in each (there Mage tactic will not work, so i suppose i cannot help you much
).
4. Dont dismiss armies (not sure if it can hurt in straight way, but it really helped me in some fights when i got forest spirits (15 from mage strating army multiplied by hypnosis+sacrifice) before reaching elflands), just put them in garrisons.
5. Always have enough money to completly change your army. I needed to change it at least 5-6 times.
6. Dont expect "high" score at the end, because AFAIK its based on time spent (not killed/loss koeficient), for example mine was less than 800 when i played wo looses.
7. And last one... If you want to make such game interesting - dont use cheating tactics like greens+invisibility etc...
P.S. Killing turtle without looses is very easy. Just block her head with melee unit (cavalry will suffice). In such case she will not use AoE damage spell and knockbacks.
P.P.S. Killing "old double" is easy if you using dryads+ring+gift.
P.P.P.S. Beware of undeads, but as Paladin it can be more easy that as Mage
P.P.P.P.S. Most hard part will be dragons, i suppose... But, who knows...
Good luck and keep us informed of your progress