I think the only way to counter it is to try and get around that high energy cost, then try to force the enemy into repeatedly discarding those 5 cards. I think having high attacking pokemon, perhaps with a lower HP, might mean that you can do damage to it before it can get it's energy to perform any attacks.
The weakness obviously would mean that a lot of fighting type pokemon with two step evolutions or even just basic pokemon like Hitmonlee could offer a serious threat if they can get out in time.
Of course the starting move does mean that it's very likely that Guzzlord is going to be able to get off his attacks faster, but he does need to discard about 8% of his deck every time he wants to use it. In which case a mix of pokemon like hitmontop and big bulky defenders like the new Dragonite and Lunala could offer some counter to his moves and dish out some damage in return. As well as pokemon that will remove energy cards from him, forcing the enemy to sacrifice more of their deck.
Start off with a set of pokemon like Hitmonlee which can attack with reasonable damage straight off the bat. As soon as Guzzlord is ready to attack, send in bluky pokemon like Dragonite and Lunala and try use these to take him out slowly, trying to use trainer cards to remove his energy cards to force him into eating through his deck.