Kingdoms of Amalur: Re-Reckoning

The achievement unlocks when you kill your 25th enemy with a trap. Enemies that are killed by traps and mines in the environment count towards the achievement even though you took no direct action to result in their fatality. There are two other main ways to get the achievement.

First, traps are a specialty of the finesse class. When you invest at least one skill point in frost traps, you can plant up to four traps at once. They do minimal damage--34 points at level 1 and 70 points at level 5. Even if you increase the damage by acquiring Icy Explosion--an ability that increases ice trap damage--the max damage you can do is 110 with level 5 ice trap and level 7 icy explosion. So, you would need to whittle enemy health down before luring them into the trap.

In contrast, the tier 6 ability Gambit hurls up to 18 explosive devices that land in a small area. At the lowest level, they do over 1400 damage and, at the highest level, they do over 2500 damage. If you go to Odarath East, you can dispatch a few groups of Boggarts in minutes using this technique and unlock the achievement. Another benefit is that these kills count towards the Shock and Awe & And Then There were None achievements for killing 100 & 500 enemies respectively with abilities. The downside is you cannot unlock Gambit until you have invested 70 skill points in Finesse.

Note: If you are using the finesse ability traps; taking lesser, greater, or master damage boost potions will increase the damage dealt thus increasing your chance of getting trap kills that count towards the achievement.

If you are not using the finesse class (primary, hybrid, or jack of all trades), the second major way of getting the achievement is during the Teeth of Naros questline and doing the side quest: “Of My Own Invention.” It is most efficiently done in conjunction with the questline’s main quest, "Shapen in Iniquity." Right after accepting Shapen in Iniquity from Secandra, go to Idylla’s Living Quarters. Speak to Pagus and accept his quest—Of My Own Invention. Return to Secandra and follow her. Speak to Anokatus when she tells you to. You must recover Arkes’ Circlet.

Follow the quest marker to enter the sewers as indicated by the amber arrow. As soon as you enter, make sure Of My Own Invention is the active quest. Follow the quest marker west. If your detect hidden skill is high enough, you’ll clearly see the traps Pagus wants you to test. Note that the one on the east is a combined ice mine and spike trap—this is the most lethal of the three traps. The one in the center is a sliding blade. The one to the west is a spike trap. The lever that activates them is just to the north of the traps. The traps will not affect you. They must be activated by levers.

It’s just a little tricky to get to the levers. You must go a bit south to a sewers ladder. Once you have ascended, you can go north directly to what the quest marker calls a lever. You'll note that it is not a single lever but an array of three levers. Facing the traps, the one on the left corresponds to the one with the dual trap. So it is a little easier than the other two. However, you can use any combination you want.

A good tactic is to use the lever on the right to trap the lead rat. Then, quickly move to the left-most lever to trap the rats behind the leader. Each lever requires a second or so to reset. So, you can't just spam. It doesn't matter if you let a few rats escape--there are plenty more. You will satisfy Pagus’s requirement before you unlock the achievement. So, continue trapping rats until the achievement pops:

It’s not difficult but there’s a slight learning curve. So, it’s possible you might not trap 25 rats before they stop spawning. This is not a problem. You won't have to reload a save to try again. Instead, when you return to Pagus to collect your reward, he will ask you to trap more rats. The rats will begin spawning again and you can use that opportunity to unlock the achievement. The below video demonstrates killing 25 rats with Pagus's traps to unlock the achievement:
Achievement Tracker: You can always check your progress using the achievement tracker. In fact, before you start deliberately working towards the achievement, if you pull up the achievement tracker, you might notice you have made some progress. This is because you have been fighting enemies in the vicinity of traps. Any enemy killed by a trap that was in the environment is credited to you. So, quite likely, you have to trap fewer than 25 rats for the achievement to unlock.

This can be easily obtained once you begin play in the Teeth of Naros DLC area. In the "Of My Own Invention" side quest, an NPC by the name of Pagus will task you with going down into the sewers to use a series of traps he set up to eliminate a bunch of rats. The rats waltz down a little path while you stand up on a platform and use three switches to activate the traps. There are more than enough rats there to get the achievements. Just try to be accurate with the traps and maximize the number of kills you make with each use, as each trap does have a small cooldown time before it can be used again.

This can be achieved by using the Frost Trap skill in the Finesse tree, you'll need at least 5 points put into Finesse to unlock it. The skill itself won't do much damage at first however, simply place a trap once an enemy is close to death and lure them into it. It has an area of effect too so you can kill multiple with it if they're all close to death.