Potions are stored in your inventory and only limited by that on the amount you can carry. Collect fazress potions from daemons as you kill them, run up to a portal and click A as many times as you have potions. Stage 2 - as before except the pillars ability to produce blue circles will diminish over time. The quicker/more kill the quicker the progress bar will fill! Sometimes you will get dazed like in Demogorgon, run into the blue circles around the pillars to remove this! The circles appear at random so you may have to run round the other side of the map. Try and stay as a group and kill together rather than separating. These will spawn from the purple portals (no need to taunt), just kill as quickly as possible.
Not gonna say or insist you need a buff, a heal, a tank etc this is possible without any specific team mix, the important thing is you all work together! I grouped with randoms so team composition can vary, but we had 4 DPS and 1 pally. Whether this was intentional or a glitch I don't know but it seems you can get Gold without getting Gold on all 3 sections. For me, Gold popped when getting Gold in the 1st and 2nd section and Silver in the 3rd (Silver was 3/4 bar full). Before going into the guide a couple of pointers!