This achievement requires you to use the Mage to get 1,000 kills. You should level up your strongest wand so you will be able to do a good amount of both magic and physical damage. Lightning Strike is a helpful skill and using it will help you get kills, especially where many enemies are in one spot. Enchant skills such as Fire Enchant are helpful as well. With some of the elemental enchants, you can hit + at the same time to shoot out a larger version of the element which does more damage if it hits, but also costs more AP and has a reduced range. You need to time it correctly or you will cast the spell instead. Try to use team skills often as they can help you get a lot of kills. If cast at the right time, Tornado can be your most dangerous skill. The down side is that it takes five people to cast and it can't enter or exit the castles nor cross over bridges. Meteor Storm can get you some decent kills if enemies are grouped together, but the down side is that it takes four people and the landing spots of the meteors are sporadic. Also, note that the Warrior's Super Guard skill can block meteors. Lightning Arrows are by far the easiest chance to get kills because it only requires two people to start and has the quickest recharge time. The down side is that the arrows are scattered; you only get credit for people you actually kill unlike Tornado and Meteor Storm where you get credit for all kills, and if someone hits you while you are shooting your arrows, you will get knocked out of the skill.
Thank you AOS Puck for the information!