I can think of a few methods if you really want to try. I haven't actually tested either one but you can try them out pretty safely without harming your AGC (more on that at the end).
The simplest one, which is the less likely one to work, is to always play the game in Offline Mode and see if it will affect your AGC.
The one more likely to work is to block access to steamcommunity.com in your firewall while you're playing the game you wish not to be counted towards your AGC. Why am I suggesting this? Well, because I've experienced this first hand with a game that I DID want to be counted towards my AGC and it DIDN'T. Once, I was playing a game to get a certain achievement (which was not easy to get either), and even though I had done what was necessary for that achievement, I did not receive it because SteamCommunity was down at the time. So I had to wait for SteamCommunity to come back online to play for and obtain the achievement again, for it to finally be registered on my account and counted towards my AGC. Which was very frustrating btw.
And the one that I think is guaranteed to work is to create another account that you do not care about and use the Family Library Sharing feature in Steam to share your entire game library with that other account. This way you'll be able to play any game in your library, you don't want to be counted towards your main account's AGC, on your secondary account.
So, feel free to give the first two a try (if you really want to play on your main account at all times) on some achievement that's very easy to get, on a game that is already counted towards your AGC so you won't mess it up for no good reason.
PS. I understand why one would want this and NO, it's not cheating. I too have games that I absolutely suck at, but that I would like to play anyway without wrecking my AGC. Doing this will, by no means, give me a 100% AGC because there will always be games I'm good at and which I enjoy playing, that I will not be able to obtain all the achievements for. And that's ok. But I would really like for my AGC to reflect my performance in games that I really do spend time on to collect as many achievements as possible and not games that I merely toy or experiment with.
Update: I just tried option #3 and I can confirm that it's working.