The Epic Event of Mountain Vargath is tricky to use and requires setup but is very powerful, and arguably the strongest economic card in the deck. One of the big ways to gain an edge in summoner wars is to not allow the opponent to attack your cards, and Stronghold can take 3 targets that can be careless in their positioning and put them outside of harm’s way. That is a very powerful effect for just 1 card.
However, after that tricky part happens and you save your units, the Mountain Vargath now need to restart their march back into enemy territory and for this aggressive faction, that can be a major downturn. 
Ramming advance is a way to cheat this, but what most games will do is position gates in such a way that you can use Stronghold, save your units and be ready to quickly rejoin and push again. So this is an event that requires players to both think about execution and set up the whole board state. Learning where to place gates in a way that your Stronghold use(s) saves your units but doesn't halt your pressure, is a very important skill. Sadly that is not something this guide can easily help with as each matchup and game is different and different gate positions are optimal in different situations, but it is something to have in mind when playing the Mountain Vargath. Don’t take the easy route of discarding Stronghold if you don’t know what to do with it, it’s a very powerful event, arguably the strongest you have, if you manage to use it properly to maximize the Mountain Vargath strength, then every game you should play at least one copy but most likely both of this event.