In general, find a spot that has a few boar spawns or at the very least a mix of spawns with boars as well. In some cases, spawns seem static (ie 1 type of creature spawns at that respawn spot like necks in a creek or the like) In other cases they can see to be a semi random rotation or what spawns in that spot and depending on certain conditions(rain spawning more necks etc).

Once you find the spot kill everything, leave the rendering distance and come back in a bit when there are respawns, rinse and repeat until you get your 2star boar. It may take days of in game time but if boars spawn the chance will come.