Ah, bonus experience. People will do crazy things from bonus experience. They might drive themselves insane with Arcane Research, risk catastrophe for everybody from Delve Too Deep or show off their death wish with Charon's Obol. However, how possible is it for someone to slay a giant pile of monsters?
The first and most obvious caveat is that you have to be a monster hunter to hope make this work, and that you must also have access to the Rogue pool. While this is a restriction, it's not one of the heavier ones - a number of rogues can be reasonable monster fighters, and a number of non-rogues have enough access that it's not too hard to add one copy of this.
Second, this means you need to be able to kill 6 HP worth of enemies in one turn. Practically speaking, this generally means that you'll be using it after either finishing off a single large enemy, or a pair of decently chunky enemies. This ends up being a more notable restriction. In smaller games, this is only likely to happen in very specific circumstances. In solo, the only way you'll have 6 health worth of monsters in one turn is if you've had a particularly rough set of turns and failed to finish off a monster along the way, both of which mean you're already in a less than ideal position. In two player, it will require appropriate setup, since a teammate might leave a hunter in place and you can try to arrange for it to approach. It's when you hit three investigators that this becomes merely 'unlikely' - it becomes more plausible that you see multiple monsters. Some guardian tools like First Watch or On the Hunt help to find monsters, while Dynamite Blast can often kill them.
(The primary exception to this is if you're about to hit a scenario where you know an elite monster is coming up, and you want to get into a slugging contest with it. In solo games, elite health might end up on a borderline. However, in two or more player games, elite monsters almost always have enough health that you can try to finish one off.)
Third, in scenarios where you can't finish something off, this is only worth a single symbol when committed. It's a very weak commit option, so you'll only want to include it if you're fairly sure you can use it.
Fourth, it's worth noting the card itself is not fast and ends your turn. For most characters, this means that your final action must be spent to actually play the card. This gives you all of two actions to obtain the required kills, unless you're using Leo De Luca or have some other method of obtaining bonus actions. It also means that you won't be managing two copies unless you manage a second unlikely fight on top of the first unlikely one.
Finally, it's worth noting that the net result of your setup and sacrificed card and action, the reward for this is a single XP for one investigator. All of those conditions end up only helping a single person. This usually means that your entire team needed to coordinate to set this up, and the result is that you spend a single action and card to get a single XP for yourself. Is that much effort going to be any more possible than gaining another XP for the entire group for meeting another victory condition? More often than not, I think the group would rather go for another victory point outright.
With so many different restraining conditions, is this worth it? I think one copy could be playable in a few very specific characters or swapped in with Adaptable when you know there's a good elite target coming up. If a fight occurs in the first mission of the campaign, you might also be able to put it in your initial deck and replace it early. Otherwise, this card is too inconsistent to be worth the card slot. XP is valuable, but I don't think a few bonus XP for one investigator is worth permanently forcing your entire group to play awkwardly and the dead draw it would otherwise represent.
(TL;DR: Probably not worth it. Use Adaptable for elite fights or drop it early. Only leave it in if you're an extremely focused monster fighter in a larger player count game.)
