Sarizaddi :
After weeks of nailing down this exact pop circumstances, I believe I have the most positive understanding of this NM.
Black indiates actual walls, Blue and Yellow as described below.
Alastor Antlion is a 2 hour plus lottery NM. In the above map, excused the slight inconsistency. The yellow dot depicts the spawn location of a Trench Antlion. Most noticable details about this room is that it is a small alcove with a Trench Antlion spawn, and the rear of the room is cut off by a black geyser making this room a dead end. This Trench Antlion and this Trench Antlion alone is the only Trench Antlion I have ever gotten to spawn him, so I sincerely believe he is the single only placeholder. I have farmed the entire area and am aware of all 5 Trench Antlion spawns, and I have camped them religiously, some days concentrating on one single spawn, sometimes the entire area of spawns. All seven positive pops came when the Yellow Dot Trench Antlion did not repop as expected.
As a method to hunt, it is extremely simple. Camp the yellow dot Trench Antlion exclusively and religiously. The repop time is approximately five minutes. The Trench antlion is DC to level 75, and easily soloable by one or two people. You continue popping and killing the yellow dot Antlion until one time he doesn't respawn as scheduled. Sure, wait an extra five minutes or whatever makes you comfortable saying that the Trench Antlion did not repop. Even after waiting two hours after a spawn (or depop) the Alastor spawn is still lottery, meaning it might be five minutes or it might be 8 hours. Take note, though, Alastor
is not a 21-24 hour NM.
Once the Trench Antlion does not repop, that is your que that Alastor has spawned. The areas marked in Blue on the map are cracks. Alastor can be in any crack in this area, though he is somewhat local to particular cracks. To spawn Alastor, simply rest at any of five major crack points without sneak on, or as it suits you rest and move around cracks until you find him. One crack is across the geyser wall, but basically Alastor is confined to what I call "the big room".
Alastor is not soloable, I don't care what braggart ninjas and red mages say. He hits for an average of 150 to 250 damage per hit, double attacks frequently, uses a poison spray that goes through blinks, and has a TP attack that can do upwards of 1600 damage going through blink. His life is as above, probably about 3000-4000 HP, which isn't much, but the odds of a solo person doing that damage before Alastor gets his TP move off to one shot you are slim.
I would safely say a minimum would be three people, an easy kill with a well-balanced group, and major total overkill beyond that. Being that you do not have to travel around to pop the placeholder, this hunt can be done at any time, day or night, and weather, season, moon, element, and day of the week are all inconsequential. The Jupon is in fact a 100% drop.