Try to keep your distance from Grux, but if he charges you, he will usually follow with the ground slam that pulls people towards himself. I save my teleport to get out of range of that.
Grux is a ziemhlich Mana dominant champion. at 3 skills so this is already low mana. uses it, most playing Grux, this never mind and that is their downfall. If so Grux attack your tower before it will have Pushed the Lane and have thus been wasted already Mana. this is precisely then your chance to kill him. Excuse my English, this is the Google translator program :)
Hit him with a few rocks, then Ulti when he gets close. If Ulti doesnt kill him, tether him and hit him a few times while you wait for your rock to reload, at this point he should, if not already, be dead, so hit him with another rock FTW.