Same. 400 goblins and a total of 650k monsters killed, zero rainbow goblin. I was mad at first but then I checked gobo leaderboard to roughly calculate the chance of rainbow goblin spawning and concluded RNG was just not on my side. Gave up weeks ago and now awaiting 2.0 to come.
Singularity + Doom combo have kind of insane damage potential. But, in reality, the skill is quite underwhelming if not straight up garbage given almost all bosses are not stationary so you just end up spamming Shuriken + Doom instead