Naturally, constructive pointers are the way to go; though when in battle (it's an arena, after all), it can be a little difficult to spell out pointers in a fashion that aren't prone to being misread as rude. "Push here?", "Let's team up!" are easy to understand, but "Don't buy priests." is much more misleading - I am after all intending to say something along the lines of "For future reference, it's generally considered a bad idea to buy priests early, as they feed the enemy with XP.". I'm thinking of assigning the priest memo to a macro, but some of the errors done by new players are much less generic.
I don't know how many players actually read the forums, but I guess that's a good pointer too.
I however doubt that yelling at a team mate would accomplish anything in said game; if they don't listen to reason the won't listen to anger issues either.