Regarding the snipers, I just accumulated them from the beginning of the game. I have 2-3 intermediate saves for each turn; I can share them. I'm playing this map for the second time. From the early stages, we have 2 castles, then we get troops, and then Legionnaires are stationed in each castle from the first few days. Also, I might have captured Elves from Ivor and Olvin before they had a chance to buy them back. And I also have troops from the Steel Harbor islands, as far as I remember.
However, I didn't have enough wizards to take out a stack of units with a single shot, and I didn't think to use any tricks. I also completed the quest for 200 wizards on the swamp, which meant I was without the necklace and the sniper's bow for a while.
Another hint: if you don't have enough wizards or devils to open the gates, they might be in the refugee camps; I check them occasionally.
Added 3 minutes later
So far (I've reached Pearlwing), the most difficult battle is against Nila. I managed to win without any losses, but I didn't figure out the tactic right away.