If you are like me and haven't put any money into the game and play with people who've spent tons of money and played over one thousand hours then this is for you. Ambush kills are pretty hard to come by if you're playing against elite level 60s all day, your only hope is going for the AI, who are unfortunately also going to be better than you.
Most games I play I lose which in the case of this achievement is fantastic. Once the enemy is attacking your gate make sure you are inside you castle and hop on either the far left or far right ballista at the front of the castle. Since the AI are quite stupid they will keep attacking your gate even if they are being shot at. Aim at the ground towards their back and fire away.
This is the best method I've found trying to go for this. The thing you have to look out for are real players who will either try to mage you down or do a charged attack and destroy your ballista. In reality, you can only really get a few kills per game using this method since the enemy team is usually pretty quick at breaking down that castle gate and destroying the tower.
Best of luck, hope I could help!
EDIT: Also another tip, once the enemy has broken inside your castle switch to the warrior class and attack all the AI destroying the tower. They always rush the tower and they won't stop destroying the tower even while getting hit. So essentially you want to lose every match if you want an easier time getting this achievement.