Tactful ways to inform people that they're playing badly?

[KageTokage]

I'm genuinely interested in what you mean by a person you don't want to play with fairly often

There's a selection of individuals from my DC I personally dread having to do ES1-4 reclears with because they've proven to me even since back in SB that they're...less then reliable and prone to screwing up randomly at pretty much any point in a fight.