If you fare well against the Ace AI, you most likely will still get your butt kicked online. AI does the same tricks over and over again, one of the most predictable AIs I've seen.
And keep the following quote in your mind:
"Speed is life" 
And as Hunter already said, know your plane & know the enemy. There's no one tactic for all planes. Some are good in turn fights but have too slow climbrate for vertical fights, some (190 & P51 to name some) rely heavily on speed.
Just keep practising.
