Pick a mono-colored kingdom in the color that you want. For example, Stormheim is a mono-blue kingdom. It will only drop water-oriented traitstones.
Now you want to make sure you get the most Minors. If you Explore, you’ll have higher chances of Arcanes and Celestials. That will make you get minors slower. So you want to do challenges for Stormheim if you want Minor Water traitstones. It’s possible the overhead of dealing with the map screen each time slows you down enough to make it quicker to do explores. I don’t know for sure how this is.
But, speaking frankly: if you are early enough in the game that this is a team you use “because it kicks ass” instead of “it’s fun”, don’t go farming traitstones yet. You would do better farming gold to level kingdoms, or souls to level troops, or arcane traitstones to use on Mythics/Legendaries when you inevitably get/craft them.
Also: this is a good reason to not trait things willy-nilly. I’m only about 50% to having all the Minor traitstones I need. But I have never been unable to trait a troop due to lack of minors. Why? I don’t trait things for fun outside of a budget. If I don’t have 50 runics left over, I’m not traiting anything unless I need that troop. I find that shooting for arcanes has given me plenty ample a supply of minors.