You should MoveTo the slot position, not the player position.
Avoidance works as long as the obstacles are “along the path,” but won’t make the NPCs arrange themselves well for an actual melee fight.
Slots are not built into Unreal, because this is a particular design you may or may not want to use. If NPCs only ever fight the player, you could have them look for some slot allocation interface on the player character; if they can fight each other, you might want to put that interface on any potential target.
If the size of the meelee swipe (which was your initial request) is 3 meters in diameter, then you need to arrange the slots more than 3 meters from each other, in a circle around the player, such that they can still reach the player themselves. Whether you can fit 2 or 3 or 4 depends on how far forward the attach reaches compared to sideways.
For an area of attack, where you could have things like grenades on the floor just as well as flame thrower area of effect, you might be better modeling it as “areas to avoid,” which would be some invisible actor geometry your flamethrower NPC would spawn in the world where he expects to fire, and any NPC would treat this as an obstacle and avoid it. Whether this is a “dynamic navmesh obstacle” or something more specific, depends on what your gameplay needs. These kinds of system tend to be quite game specific (together with “cover” and “flanking” and all the rest of the hard-to-build AI behaviors.)
This is why it’s hard – you usually have to do it yourself, because every game (and every attack modality WITHIN every game) ends up with their own requirements, and they all interact.