Vikings are already very good in defense if you have both boosts. If you only have the Elite OR the Conquest boost, Vikings are still good.
The top alliances are already using Vikings in their defenses pretty effectively.
In offense they can be used effectively if you have both boosts. The Shield+Battlecry (Conquest boost) are very usefull.
The downside, from what I’ve seen, is that it requires very specific combos for Vikings to work and sometimes they’re only usefull as insta troops.
Buffing the offensive Viking would make it’s usage rate go up.
Viking’s Ice weakness is really only relevant when talking about the defensive Viking. It makes them very weak against Frosters (super popular right now) and Fritz.
Right now they have 100% ice weakness. Instead of changing it all the way to 100% ice resistance, I would suggest just removing (or reducing) the ice weakness for now and see how it goes.
When balancing, it’s always better to make smaller adjustments than bigger ones.