Oh, Arrows from the Trees. Such an awesome effect, but too restricted to see use. Let's go through your text, line by line, to see exactly why you see such little use.

First, your timing restriction: "Play only after making engagement checks." While this is fair and part of how you work, it also limits you, making it so that you force a player who draws you to wait at least half a round before doing anything with the new card they just drew. Ranger Bow may require them to exhaust a character and take up a restricted slot, but they can trigger it whenever they wish for a similar effect.

Second, quite possibly the largest nail in your coffin; "If no enemies were engaged this phase." This, Arrows from the Trees, is what makes many instantly dismiss you as binder fodder, the Achilles heel that cripples you. Depending on the quest, it can be difficult to have 1 deck manage to avoid engaging enemies during the encounter phase, but having to have multiple do so in multiplayer is almost an impossibility. Looking at the AP you came in, EACH player in multiplayer would need to keep their threat below 25 in order to avoid the possibility of a forced engagement disabling you, far from an easy task. Now, you can be used in solo, at which point it's easier to control threat, but that brings its own problems which I will discuss later.

Third, after all those restrictions have been met, what do you do? "Deal 1 damage to each enemy in the staging area." 1 damage. Waiting until the encounter phase, making sure everyone's threat is low enough to avoid engaging enemies, not optionally engaging anything (a tough decision to make, in some quests), spending a resource, and in exchange we get a delayed Thalin hitting each enemy in the staging area. That is not without merit, but it certainly seems a poor exchange. Fortunately, this isn't all you do, as the players as a group may spend 3 resources to deal an additional 2 damage to each enemy in the staging area, for a total of 3, as much as Forest Patrol! This is enough to destroy most of the enemies in the game, and in exchange for this great boon to combat all we need to do is:

1. Have enough enemies in the staging area to make it worthwhile to play you (> 1; else, Forest Patrol and Traps would serve us better)
2. Make sure that every player's threat is low enough to avoid engaging said enemies
3. Also make sure that there are no enemies in the staging area who would survive that we want to engage (i.e.; enemies with high archery values)
4. Do so with the two spheres with the least threat control in solo or make sure that multiple decks, one of which includes , keep their threat below every enemy's engagement cost
5. Spend 1 and 3 resources, either with one deck in solo or multiple decks in multiplayer (which, once again, brings up the problem of maintaining multiple decks' threats)

If we manage to keep at least 2 enemies in the staging area, if we manage to keep our threats low enough to evade them, if there are no enemies who would survive who we wish to see gone, if we manage to do so despite poor access to threat control, and if we manage to have enough resources to spend, we can make good use of you. That is four too many ifs for most players, Arrows from the Trees. You have too many restrictions to justify your effect, especially without spending those 3 resources. You might be effective if everything goes right, Arrows from the Trees...but how often do things go right?

One final note: There is a combo that helps redeem you, however, though it requires either multiple decks or -resource icon granting, like Song of Battle. With Advance Warning's help, you become much easier to use, effectively saying "Spend 3 and 3 resources to deal 3 damage to each enemy in the staging area." Your arts even go rather well together. This isn't enough to make most players take you out of their binder, sadly, but it does give you a niche use, and a niche use is better than none.

Amusingly enough, I also think that you have some use at Gencon or other more-competitive games, precisely because of how restrictive you are. So restrictive are you, Arrows from the Trees, that the chance of you being successfully used is nigh a million to one. And, as Terry Pratchett said, "It's a million to one chance, but it might just work."