Tips on being a mentor but not sounding bossy

[DevilsDontFly]

I'd say the best way is to not blur advice to the point it sounds like criticism. The line that divides those two things is pretty thin.

At the start of the duty it might help new players to feel more confident and less nervous about messing up with phrases like: "Hello! If anyone needs help with mechanics or anything else, please let me know."
That welcomes the player to ask for help if they want to. Sometimes even people that mess up don't like to receive help and preffer to learn things on their own.

In the end everything changes based on how you phrase yourself. Per example, if you see a Conjurer or White Mage that is brand new, casting Medica for single target heals. Don't call the mistake as common sense, even though it might look like common sense to you and everyone else. Make the player feel like he can relate to you and vice versa.
"Hey, playername, I remember when I started to level up healers I caught myself using Medica like that too. If you use just Cure, it won't be as punishing for your MP and it's also stronger for single target. Have you tried it?"
Even if you haven't done it, sometimes a small "lie" can make whoever you are teaching to feel like everybody makes those kinds of mistakes, even the almighty mentors at the scary end game community. And can make them feel like if you made a mistake similar to theirs in the past, tomorrow they might be someone like you mentoring new players as well.
And if you feel like this approach is not helping, then try just asking. "Hey, playername, would you mind if I gave you some tips? It might help you a lot in this trial/dungeon/fight."
Mentioning that you feel like your help is just going to impact this one trial/dungeon/fight creates the ilusion that the player isn't messing up with anything else but this one encounter. And if they are willing to listen to the help, it'll end up impacting the way they play altogether subconsciously.

Not sure if this will help you, or even if it would help anyone here. But when it comes to me, it helps, and it also helped other players I came across playing this game and many others. To me the key to being able to help someone is to make them feel like you also needed help at some point, while respecting their bondaries if they feel like your help is not what they need.