You can boost this achievement as the other 2 solutions suggest, but it's not hard to score aces in career even with fairly low serve and power stats (my character finished up with 65 serve, 50 power and this method worked long before I reached those) and will save you the time and the boredom of boosting if you're playing through the Career anyway.
You need to practice the "double risk" serve using both the LT and RT buttons. What works for me is to hold RT and my chosen serve action buttons then quickly tap LT and flick the stick in the direction I want to go just as I let go of the action button, keeping RT held down. The precise timing is dependant on the service motion your character uses, generally speaking about a fraction of a second before the ball reaches the striking zone of the animation.
Once you have that down you can hit an ace pretty consistently by exploiting one of the unreality aspects of the game: serving from very close to the tramlines.
With a right handed character (everything is opposite for left handers) when serving from right to left move as far as you can to the tramlines and perform a slice (using X) double risk serve into the left hand corner. The angle you get from your position plus the side spin off the bounce of the sliced serve guarantees an ace against most AI characters - even on hard difficulty.
Serving from left to right will net you less aces, but you will get some. You should use flat (A) serves from this side, however if the opponent is standing very close or preferably over the tramlines you can try moving your character to the centre line and performing a double risk slice serve down the middle, again it will kick to the left off the bounce and there's a decent chance the AI won't make it.
This serving method makes getting the Hard achievements cake as you're practically untouchable on serve, even if the AI manages to return the serve, 9 times out of 10 it's an easy put-away into the opposite corner.
If you happen to miss your 1st serve then RT alone + a flat serve (A) aimed directly at your opponent has a good success rate for a safe 2nd serve ace as they'll often dodge out of the way rather than attempt a return.