In game there are always people killing Zak, Krexel, HT, Wulin Yaoseng, and BF/HH. Even Pap is becoming more popular recently with the exp buff that it received. However, how come you don't really see anyone killing Pianus? Is it worth killing Pianus?
All the bosses you mentioned either spawn at will of raiders or there are hundreds to hunt (BF/HH). Pianus is a bloody irritating area boss to kill with the wep cancel spam + dispel. Respawn time for each head are left- 24 hours, right - 36 hours. Left head drops SE 20 Right head drops Rush 30 (300m+?) All the factors combined, usually there are only few dedicated people who hunt them down but with specific timers; similar to Lyka hunters. Casual players usually don't even bother searching for one because chances of finding one alive is so slim.
I think people only hunt pianus for the cert, which is used to get the badge for ht and for minature pianus when they feel poor after owling for them in the fm
Hero's will quest. Normally one needs a couple to collect 40 etc pieces during the quest as time limit is rather short compared to spawn rate of mobs that drop the etc.
Fyi while i was doing hero will as bishop, i only got 39 stones using 2 pianus minature. Got to buy one more mianture for the last stone. Rip