This should be your last achievement in the game. I was getting around 2000-3000 Kills per playthrough if I was making sure to play plenty of islands so that I could rack up kills.
When pushing for beating the game on hard (Folk Hero Achievement), you will likely prep for that by running through the game multiple times to get used to the strategy needed for winning the later islands and collecting useful starting items and traits (Ring of Command, Warhammer, Warhorn, and Heavy Weapons were most useful for me). Feel free to keep playing the game as intended if you are having fun with it or getting prepared for a run on hard. If you end up in my situation, you will likely have only this achievement left and be wanting to wrap things up as quickly as possible. Just running through the game again and again isn't the fastest way since the earlier levels have so few enemies and the later level require more time and effort. There is a nice way to boost kills, but you need to be aware of some details so you are maximizing you efficiency while not losing any progress. Let me explain:
If you find and island with a good number of basic non-archer enemies and an hill with a single ramp to protect (the one I used had 2x ramps near each other), you can simply set an upgraded Pikemen (or multiple Pikemen) to guard the ramp(s) and win the round without much effort at all. If you have turned level restarts on as a setting before the game begins, you will be able to play the island over and over and passively rack up the kills while barely paying attention. (A single ramp is not required if you dont mind setting up multiple pikeman units at the start of each round. The basic melee enemy is the key)
On normal, around level 25, I found a suitable island that gave me about 115-130 kills per round. 150 Kills is 1% of our achievement so in my case, I would get slightly less than 1% each round. Your boosting efforts will take much longer if you settle for an island with substantially less than that, I would recommend looking for an island that supplies at least 100 kills per round. I need to mention that if you are prepping this ahead of time, you should be looking for the perfect island while doing your initial playthroughs, pausing your progress on that save file, and keeping that save file intact for boosting. Islands are random, and finding the right one to use could be frustrating if you arent coming across one that works.
Simply put, when your progress bar reaches 100%, your achievement will pop. The problem you may run into could be wasted effort if you are boosting this at different times. There seems to be a problem with the game "saving" your kills when you boost the same island over and over, and then quit out to continue the grind later. This can lead to your achievement tracker appearing to be frozen or glitched. My achievement tracker was stuck at 88% for a while. Rest assured that your achievement has likely not been glitched, but you have lost some progress toward the 15,000 kills. Your tracker will never go down, but you will need to regain the progress that you lost to see you achievement progress continue.
To avoid frustration and wasted time, if you would like to boost this with the method above that doesn't require much effort, I would recommend planning to do it in one sitting.
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