Tips on being a mentor but not sounding bossy

[Vandril]

That would really only work for questions about the dungeon though. The problem is the people who won't ask questions about their class or role because they think they're already doing it right because no one has ever told them that they're doing it wrong.

Example: I encountered a Warrior once who kept losing aggro to large packs of mobs. If the pack had more than one mob, I fully expected him to lose aggro on several of them, and sure enough, a few seconds into every pull most of the mobs would go tearing after the Black Mage. I started watching what he was doing and realized he wasn't using Overpower or cycling through targets with his enmity combo. After [politely] pointing out that weaving Overpowers would keep him from having to chase after errant mobs, he said that he always assumed that mobs "suddenly" losing aggro was merely a mechanic SE added to make dungeons more difficult. He was surprised that no one had ever bothered to tell him he was doing it wrong, especially when it was such an easy thing to fix. He started weaving Overpowers into his rotation and didn't lose aggro again for the rest of the dungeon.

They won't ask about something they think they already know or don't know that they don't know it. If they're doing something wrong and you know better you should say something.

I really love this story.

Refer to my sig quote. The Dunning–Kruger effect may have been debunked as an absolute, but the correlation it describes is still very true. If people don't know how to do something right, they likely also won't know when they're doing that something wrong without some sort of feedback; either failure at what they were doing or someone telling them there's an easier way to do it. This applies to a whole host of activities, including playing a class on an MMORPG.

The story you shared portrays this cognitive bias perfectly. The only reason the WAR didn't improve on their own was because they weren't aware there was improvement to be had. And the moment it was explained to them that there was an easier way to handle multiple mobs at once as a tank, they immediately picked it up while probably experiencing a sense of "Eureka!"

As for how to approach people as a source of help, I agree with this thread's general sentiment of being polite. And there are some general tricks in wording that can help avoid triggering people:

1) Use soft language. "If you use X, it can make Y easier to tank" is soft language, as opposed to hard, commanding language like "Use X because it makes it easier to tank." Soft language adds a sense of choice to your statement, so it doesn't feel like you're commanding the other player to do something or trying to force them - most of the time, being commanded isn't taken well. So statements like "You can", "You should consider", and "If you use" are soft language, and can help avoid triggering a defensive "I don't have to do what you say" reaction.

2) Avoid "you" statements. Statements that start by pointedly calling out the other player by starting with the word "You" is often subconsciously seen as confrontational, and may make the person defensive. Setting them on the defensive isn't going to help you teach them. Just avoid starting your statements with "You", "the tank", "the healer", "that dragoon", or anything else that singles out another player as much as possible. Example: instead of "You look like you need some help", you could say "I'm willing to help you, if you're interested." Notice the combination of avoiding the you and using soft language.

I was gonna post more, but I'm running out of time. These should get the idea across. In general, you want to avoid making the person you're trying to help feel defensive or pressured. Most people will open up to your help in the form of suggestions, then.