Like others have said, it's really in how you present the constructive criticism. Let's give two examples:

Unhelpful: "DPS is SLOOOOOW! Step it up, and git gud."
Helpful: "Hey, how's it going? I noticed you weren't using Heavy Thrust. If you work it into your Rotation, you'll find you deal much more damage. Just some advice."

You'll notice that the first comment doesn't actually do anything to improve the poor DPS player. It just tells them "step it up". If you don't understand your rotation...well, telling them "Do more damage" doesn't really do anything. If you don't know how to deal more damage, all it will do is irritate the player. If you give some friendly advice, in a polite manner, it tends to work much better. It also really helps if you ask if they want any advice before just declaring it. Just a simple "Can I give you some advice?" can do wonders.

On the "Political correctness" thing, there is a massive difference between being oversensitive, and someone legitimately being rude. Saying "Git Gud or GTFO" is just rude, and doesn't really improve anything. Saying "Hello, Fire spells deal much more damage than Ice spells, and Black Mages are intended to switch between the two" is a much better, more helpful piece of advice. If the player is a jerk about the advice, well, then the best option is to honestly just be quiet, deal with the slow run and go on with your day. One slow run is better than arguing through a whole dungeon.