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

[Krotoan]

I mean...then why even bring it up?

See, if I was tanking and you said that out of the blue like that, that would piss me off a bit and I'd most likely just ignore you. That's a small nitpick and by even mentioning it you're giving off the vibe that I don't know what I'm doing/you think you're superior to me when you are not.

Because in this hypothetical facepulling may not be the optimal strategy and possibly ranged pulling would make it go more smoothly. It was an example. If you always take suggestions as personal attacks or veiled superiority complexes I don't think the problem lies in the suggestions.