Does a decoration spawn on every red or white candle? And the red candle is the "stage" right?
I used to do that spawn frequently soloing the first wave (the bats and gore fiends) to get the new level with my sampire, and he can kill smaller stuff like 20 at a time in seconds, so it breezed along.
For the deco, theres a chance that something will unlock when the spawn starts (when you challenge the alter) and for each level up. So you can work the spawn up a level, then let it decay, then work it up a level again to get the deco spawn chance over and over for that level for as long as you care to do it.
I personally found that if you don't get something to appear the first 1-3 spawn chances then its super unlikely it will spawn anything, and you need to finish the spawn, but if something does appear there's a high chance youll get something every chance or two. Not sure if its just my observational bias or if its actually something in the code though.
My advice to you would be use a samprie with whirlwind, or a necro with wither or failing that use earthquake to mass kill the gore fiends.
