(Jirachi, amoonguss, this isn't gen 6...)
I started building with the core of zard-y + rotom-w because zard-y can switch and deal with most grass types (mainly bulu, tangrowth and ferro). Also GF gave rotom defog, which is really important to keep zard healthy. Then I added zygarde because it also benefits from zard's ability of removing grass types (mainly ferro because zygarde can't KO it with z-outrage). At this point I needed a rocker, and susprisingly lando didn't fit on the team, so I put clefable. Then I needed a steel type and a grass type, I considered putting kartana, but I thought my team needed something more passive, thats why I put amoonguss. In the sixth slot I needed a scarf and a steel, so I put jirachi. Also, I didnt want to build a zard-y + ttar team, because it is too standard, and I thought a lot of teams would contain this core.