Many of you though wont be happy with that, so let me explain why it is gold only:
Points why its gold users only:
1) You need to attack rivals, this as you know uses up stamina, im ok i have 40 stamina points, however i am only triggering 1 boss for around every 20 stamina used, so i have to use 10 gold to keep filling up.
2) Boss damage requirements are insane the more you progress, for example to finish boss on level 20 or 30 (cant remember which one to be honest) required 7 in game cash hits to kill it, for every level 10 boss you progress to it increases considerably, i have just completed the level 70 boss and it took 14 x in game cash hits, this is why it is impossible to complete gold free, you can not generate enough energy to do that amount of damage in the bosses cycle again you have to use 10 gold to refill once, level 70 as i require called for me to refill twice.