I read somewhere on the internet about how to do a 10+1 roll to get a 5* or 6* ( I forgot the source ), that before you do 11x roll, go to normal gacha and do the 10x roll first. Then if you see the 3* at least 3 times in a row, or until you get a 4* or 5* (ampys), then back to do 11x premium roll. That way you will have higher chance to get the gold or rainbow.I haven't confirmed yet, will try it when I save enough 50 gems

P/s: imo, it's probably just a rumor and the gacha is all about luck. But I still wanna try it thou :P