Sacred Units!!

[HoneyBadger]

With Niefelheim, the first thing you want to do is research the spell that lets them fly. That takes care of them having to wade through chaff while you fire flaming arrows and such at them.

They are really handicapped against undead though-I'll testify to that-so the best thing there is to switch from Niefels to the regular priests until you've researched Dust to Dust.

With Jarls, you have to think of each one as a piece in a puzzle. Very few tactics are going to work against every single thing a Jarl can do, and one Jarl can have a significant effect on an entire enemy army. So build atleast one specifically to counter whatever tactic you think your enemy is going to throw at you.

Equip one for MR-give him a starshine, a lead shield, and an amulet of antimagic-you'll have a unit with an MR up around or above 30. Give another one morale boosters and berserker buff, boost a third with 2 swords of sharpness, a horned helmet, a ring of the warrior, and as much quickness as possible, so he can deal with heavy SCs. Give a fourth an eye of aiming and a storm bow so he can take out casters using area effect. Make a fifth resistant to everything with elemental armor and a snake ring, and equip him with a hellsword. Give a sixth a ring of regen and an amulet of reinvigoration. Arm a seventh with a holy lance and make him your Prophet, so he can deal with undead infestations.

Put them all in the same army, with decent support, and atleast 1 of them is going to do a lot of damage to whatever would otherwise stop you.