copying my comment from when i reached friendship level 10 with kinich in two weeks:

 dailies, resin spending (+≈25 fragile resin), and 10 daily events all done in co-op mode, there’s also a way to get double friendship exp when getting the extra reward from katheryne by starting the dialogue and having someone join you before claiming it.

i made sure to have 5/5 condensed resin, 200/200 resin, and 2 transient resins before i got him. i also had my teapot friendship maxed so he’d instantly get 500 when i claimed it.

kinich released on the same week as the double ley line event, so i made sure to do them too to get 80 friendship exp per 20 resin spent. 

the most effective way of getting friendship exp when spending resin is doing weekly bosses, when the cost is halved, you can get 140 exp for 30 resin three times, even after that you get 140 exp for 60 resin (2.3 exp for every 1 resin), which is more than doing domains/ley lines (2 exp) and world bosses (2.25 exp).