First pet to max level?

Hi all. Trying to do pet battles and level up a team or a few to max level so I can take part in event week. What's the best way to get a team up to max level? All the guides I have seen assume you have at least 1 at max level so you can use tokens and buy the level up items.

But how do you get the first team to 25 to do it? What's the best way? Grinding?