Indeed they are. I have a pretty decent team that handles most situations but those, not so much. It doesn't help that it's mostly silvers with a few golds and platinums that are not mincosted or max skill even though I have been playing for about a year and half or so...
I must commend you for trying to farm for silvers. I honestly don't have the patience for it. I tried to do it to prepare for Spica's awakening a few events ago and it was painful. I am honestly hoping to get Juno (I think that's her name) over on DMM since she increases silver drop rates which would be very helpful.