Viviric summons 2 wyverns each turn. You can kill only 1.5. The time is working against you.
Put your east healer aggressively in the forest, but outside of goblin range. Two wyverns will attack, but the healer should survive. Move your two other healers towards your heroes. Do it clockwise outside enemy range and hide them behind forest for safety. Move Arcaena east into the forest, so she can shoot the goblin.
Mission 12 - Passing the Village
Buy Warrior in the village center. This gives you view to other portal at the west. At that side, buy Bear, Healer and Archer. They will easily clear active enemies on the far west, using forest as cover. Return them to the portal, with bear in the front forest.
When your army enters the gate, it will be villagers who are ambushed. Their forces would split. Your west Bear in the forest is immortal due to healer support. Eventually fall back a little if you have trouble with Crossbows.
Your main army mages would shoot out remaining defenders. The Wizards are one hit for Mages, so be sure you do first attack.
Mission 13 - Crossing the Blockade
Recruiting inside of blockade smashes defenders, but actually you do not need recruiting anyone. Your troops can clear the blockade without getting hit. After that, enemies will attack from 3 sides.
From north 3 cavaliers. Let Sylvius immobilize one, while Arceana and single warrior kill the other. Then they can help on the east side.
Send 2 mages, catapult, warriors and healer west.
The rest to the east. Hide warriors behind forest and attack only from the trees.
Mission 14 - Unexpected Strike
No recruitment needed. Your catapults will clear the area one by one. Use Sylvius against cavalry.
Alternatively, you can take fort in turn 2. When cavalry goes away from castle. You will get access to fort portal by clicking on enemies number and rotating camera. 2 warriors, 2 archers, healer and mage would take it over.
Mission 15 - Persistence
Hide in keep. Get away from catapults range, forcing them to come closer. Put 2 warriors at each gate. Catapult at west. Target enemy catapults, wizards and crossbowmen.
Mission 16 - Mountain Hideout
Beware of cavalry coming along the road. Sylvius can block whole squad by rooting first one. Just remember not to kill it.
Crossbowmen can't shoot if moved and has shorter range than Mage. He can hit them safely.
Mission 17 - Lightening Strikes Twice
Rush against enemy to gain space. Then run away and shoot. Trees creates two choke points on the road. Hide behind them.
Your backup force must be careful about cavalry. Still no casualties needed.
Mission 18 - Elunore's Infestation
Buy 3 ballistas in the west portal. Move two of them one step north. Keep the third on portal.
Buy 3 healers in town portal hidden behind building.
Check Vyvern's line of sight before moving. Let it not get free attacks.
Mission 19 - Elunore Siege
Cavalry at the north gate are actually your Unicorns. Send them, all your heroes and one mage, to smash attack at this side. Arandur skill shall protect Sylvius and Arceana, so they kill the pike men.
Get two healers on the east gate.
Mission 20 - Scorched Chasm
You want 3 Ballistas in line. North one is already placed correctly. Move southern one into line but out of road to keep clear path. Buy third ballista. It should reach its place in turn 2. Remove units from its way.
Buy 2 Unicorns near Arceana. Try them to score 4 movement charge. Then they can chase Vyvern before it flies away.