Damage Mortar with Sylvius. On southern portal, recruit Unicorn and rush it to help the druid. Let Arceana and Archers head that site. On north portal, recruit a Healer and up to two Ballistas. Ballistas must target enemy canons first.
Round 2: Arceana finishes one mortar, Unicorn kills the other using charge. Sylvius should heal himself. Recruit Bear on southern portal to block Boars from forest.
Round 3: Recruit a Healer to heal your Bear. Sylvius alone stops mortar side Berserkers by switching and rooting targets. Your main army would easily kill anyone who passes the river under Ballista fire.
Mission 22 - Safety Line
A line of 3 Ballistas with some support would easily kill any dwarf that gets lured from their formation.
Mission 23 - Hide And Seek
One Ballista at each side. Step, shot.
Mission 24 - Grasped
Move main forces towards portal, to regroup and form a line. Recruit only Mages and Ballistas. Target Berserkers and Rifleman first.
East forces are weak, but they can shoot dwarves while running back on forest hill. They would eventually win without loses.
South forces shall run away. Ballista here can not be saved, so let it kill Riflemen and damage some Berserkers.
Mission 25 - Hammerstone Siege
Allies inside the fort can't be saved. Dwarves have overwhelming force there. In first round kill both Canons, and fallback all other forces. Recruit Mages and Ballistas, then proceed with normal sledge.
Mission 25 - Ruined Keep
Switches on walls opens gates and enables portals. They can be activated with summon scrolls. Only the last 2 are sensible, as they call support.
Destroy gates and wait till are wanderers rush out. A Single Unicorn would deal with Mortars if it moves through forest.
Remember that Sylvius can heal units within range even if they are on the other side of a mountain.
Recruit another healer when it is safe.
Attack Gatekeeper Kraassu with heroes and Mages, but note that with high bonus, he kills regular unit that are not in forest.
Mission 26 - Before the Stone
Just move slowly. Recruit one Ballista, to force orcs to leave palisade. Additionally, it can snipe a prison camp to free a Mage.
Mission 27 - Circle of Ice
Chests contains 40 gold each. Kill 3 golems left to Viviric, then attack him. Sparing or taking his life does not have later impact, as this is the last mission.