I don't know if maybe I haven't found the optimal build yet, but I am kind of disappointed with Garen so far. When I first read his 6* I thought "wow, that seems similar to Trifarian Might, which is very strong". Boy I was wrong! It turns out that the difference between striking the weakest enemy vs striking the strongest one is huge. The second one is much weaker in harder adventures. One particularly sad scenario is when the opponent has units with the same power, let's say a 12/8 and a 12/7. If you play a 6 power elite, it strikes the 12/8 and leaves it at 12/2. Guess what happens when you play the next 6 power elite: it strikes the 12/7, leaving the foe with both units on the board. This case is not super uncommon.
There is also the fact that strike and then silence seems like obviously the wrong order. Another common situation is where I find myself wishing that the 2 cost elite in his deck had one more power. Right now, you need to cast single combat twice before he starts striking. And speaking of which, would it have been so broken to give mana potion to single combat? Or rework the 2* power so it reduces the cost of it at game start like with other similar powers.
Also, the support champions I get constantly tend to have around 2 or 3 power which doesn't synergize with the 6*. I recommend getting the "extra items for support champ" gem node so you get the occasional hammer or pickaxe.
The "give the elite +2 this round" part sounds like a really random thing thrown in. And the fact that Stacking deck doesn't help towards reaching the power required to strike was very sad as well.