Towers used:

Explosive Shots

Barrack (possibly not necessary)

Arrow Tower: Excellent II lvl 30

(PS I am aware it should be doable with lower lvl, however I haven't tried it before)

This map includes a lot of stealth units coming from bottom, therefore we will try to "snipe" them with explosives along the way.

Vikings zone2 map2 hard 01

Map overview

First of all, put Explosive Arrows on the rightmost top place and upgrade it fully. The second most interesting point is to put barrackts on the bottom-left place so that incoming stealth units are "discovered" at the very beginning and they will not be able to go past your main defence point. If you see that guards are busy and stealth is coming through, use your unit-spell.

When you have enough goods, clear the second main Explosive Arrows just below first one (Phase 2 screen) and upgrade it as soon as you can,

Vikings zone2 map2 hard 02

Phase 2 - Full upgrade Explosive Shots

When you are done with upgrading, put your next Explosive Arrows on the opposite side of the road to start punishing vanishing (pun not intended) units (Phase 3 screen)

Finally, you should be a point that vanishing units start to overcome you - it is no longer time to put you guys from spell there, start putting them on the "hot spot" within your gates-to-hell.

Flying dragons may try to perish your heroes - use your Thunder on them and keep damaged heroes behind the line.

Continue with putting Explosive Arrows down the same road (Phase 3 screen, tower which is not upgraded on the screen) - at this very point the creeps will slow down a lot.

Vikings zone2 map2 hard 04

Phase 3 - clearing the invisible