Yes, this skill is essential to power level. Yes, it's over powered, but who cares really. This is way better than the unopened soda bottle trick. The facts in the post focusing on increase the deadliness of the skill. There are additional things you can do to augment this skill, but it's out of the scope on this guide. Check out Land decimator in action and all of it's glory. Here it's being used to magic extract and shard farming. Nice multi-tasking happening in this video.
You can combine it with an Insta-Kill Evility
A lot of people don't know this, but you can get the added benefit with the Fatal Slash Evility. This Evility is obtained from the Rabbit class. The Fatal Slash Evility has a 30% chance to deathblow enemies with a lower speed rating. This works extreme well with enemies with a slightly lower speed rating than you. You can adjust the difficulty factor to exploit this. Please keep in mind that this doesn't affect your accuracy.
You can perform multiple attacks per turn
The Twin Dragons Evility will allow you to attack twice if your character doesn't move. The idea is to take your Sage out of the base panel and fire away. Because the Sage didn't move, you will get to attack twice. You can either pick up the Evility as a Sage, or you magi-change a monster with it. I have had a lot of success with Izuna and Drumdawn. Izuna magic changes into a staff and Drumdawn has the twin dragon Evility.
The Sage also has a skill named Multi-Attacker. This skill is unique. Unfortunately you can share it will other classes. Just create a few characters of the Sage class and that will allow you to stack attacks.
Max Hit Rate is 50%
You can do a lot of damage using this skill, but the hit rate is capped at 50%. There's not much you can do about this. This is baked into the skill. Just use skill that multiply your number of attacks.
Using this skill is the best way to power level. Take a little time to explore it. It took a while for me to make it deadly, but with a little effort you can make your Sage's Land Decimator deadly too.
There a limit on the amount of stats you can gain for magical extracts. That limit is 10 million stats. You can get magical extracts by capturing monsters, interrogating them, and extracting their essence. You can do this early, mid, or deep post game.
The capture monster squad is essential to capturing monsters. Try to raise the level of the squad as soon as possible. At capture level 3, you can capture a 3x3 block of monsters.
Try to work towards a few goals at the same time. For example, you can capture monsters as you level up an item in the item world. Capturing all of the enemies on the board will advance to the next level and provide a good source of monsters to interrogate and extract.
I have Seraphina as the leader of my interrogation squad. I used threaten for her interrogation method. It's pretty effective, but experiment with different characters to see how fast you can get them to surrender.
After you interrogated a few monsters and reduced their SP to 0. You can recruit them or extract that powers. If you don't want to recruit them, extract them. That's a better option than letting them go.
Extract Item Limit
As you can see below. The limit for a magic extract is 10 million - 1. A character can only gain 10 milllion stats total from a Magic Extract. You can give a character two magic extracts with the stats below. The character will receive 10 million stats and not 20 million - 2. Beware of this limit and don't waste your extract.
Best Place to Farm
On Carnage mode, the board of Double Fake is where you should go. There's a 100+ Asagi's there. Using the Sage's Land Decimator skill can make quick work of many enemies.
See it in Action
Will They Nerf This?
I believe the answer is probably not. This is a core component of the game. This feature is a great alternative to Disgaea games of the past. Using the Puppy paw stick and a reset you could duplicate pretty much any item. However, you didn't have a lot of progress to show using this technique. With this method, you are rewarded for playing but have a sensible cap in place.