I shall simply present what we discussed and then these ideas may evolve with time as you show them to others who may be relevant.
Now, again, I was never involved with the capture or failure to do so with this dwarf, so I expect others may have more correct ideas for certain details to approach. How ever, I imagine this will all be useful enough in its own way, if altered to the situation at hand.
Step one, once we are active and ready, draw it in. I suggest the location of his old camp that you spoke of. Mayhaps there will be some little trail left of what crimes he committed. If such is found? Cast it into the fire, and we shall see how things react. How ever, before casting anything into any fires, I do suggest we set a trap of some sort. They like to take that which is good and natural, and alter it to their own vision. We may need to bring something or someone they would wish to alter. I am uncertain on the best path here. Someone in good physical health would likely be a good choice. Though, I expect most warriors we bring along to battle it would fit this description well enough, but consider other options that may seem more tempting, if you can.
Step two. Once we have decided upon a way we might attract the fragment, we may as well use what we discussed to experiment on what works on these beings. I suggest we bring live fires and similar naturalistic obelisks as what you suggested for the larger operation, though for this smaller one. We can confirm if they do indeed contain it.
Step three, something to fight it with. I expect your blades and what not might not be a terrible option against such things, how ever it might be worth it to use this opportunity as a way to experiment. Which elemental energies appear to have any impact upon the creature, if any? I suggest we prepare weaponry that will allow us to experiment with that. An evoker of the elements might do. Then if it reacts in a particularly bad way to any of it, we may try to use that against the larger one. Of course, I know, you do believe fire will work. But I suggest we try more than just that, so we might learn.
I expect this three step plan for how we draw it in, prevent its escape, and attack it, would give us room to learn what should work once the real deal is here.
Shum Sheroy