Hearthpwn

The Grandmother's are very good for playing around AOE effects, but if I were to replace them I would use 1x Gollaka Crawler and either 1x Trogg Beastrager or 1x Grevious Bite.  :) 

Against the Warrior build you usually need to plan a few turns ahead. In the early game you need to consider the option of them playing Ravaging Ghoul on turn 3 so you should try to keep as many minions with more than 1 Health on the board as possible. (and play Grandmother if possible as it's resiliant to AOE) 
In the mid-game you should try to play around Brawl by keeping as many Deathrattle minions on the board as possible and not drop small minions onto the board if you have any big ones already. 
In the late game you usually need to kill them with burn or by hitting them with Highmanes, also try to play around a 2 damage AOE as well as you can. It can be a good idea to save Stampede until the late game to fill up your hand in the matchup. 
You can also tech in Hunter's Mark if you're seeing a lot of Taunt Warriors. 

To win against Plant Rogue you usually just need to go face with almost everything and kill them as fast as possible. (and a lot of the time you shouldn't be playing Savannah Highmane as it will just get returned to your hand with Sap, so it's better to just play a smaller minion and Hero Power) 
Also it's important to hard-mulligan for 1-drops as you won't win without early game pressure. 

The Stampede is mostly experimental, but I like it as it can refill your hand against Control decks.  :P 
Hunters Mark is good if you see a lot of Taunt Warrior. 
I'm running the double Roc with the Gollaka Crawler because I noticed that there was a lot of Pirate Warrior from 500-100 in legend.  :) 
Griebous Bite is mostly just a flex card that I choose to use because it's good against other Midrange Hunters, Quest Rogue and Pirate Warrior.  

I guess you could use it in the deck as a counter to Taunt Warriors if you're seeing a lot of them, but I generally think it's too slow.   :) 

Knuckle is less good against face decks though, which is what a lot of people are playing right now in my experience. 

I don't have Rat Pack/Stampede - what should I replace it with?

I would add in another Gollaka Crawler and Grevious Bite.  :) 

It didn't take very long for me.  :) 
I've just hit top 20 now.