Did this guy twice as our resident NINs wanted the arm. The fight is short and sweet, but the getting to the pop area is a bit of a hassle. Going through the Greater Colibri camp > Mt Z> then just run straight to Halvung will put you at the closest spot.
You can sneak/invis all the way up to the first bombs then just fight you way. The Bombs aggro to spells so its easier to just fight you way through it all. Once you get up to the last bit before his ??? is the longest part.
There are about 8 or 9 true sight trolls guarding the path to his ???, the area is right next to the troll beseiged boss. Its easier to just fight your way through the trolls one at a time.
Once your at the ??? the bombs are on a 13 min respawn, which isnt a big deal as it will take about 5 min to kill Reacton.
Notes on the fight:
1. Massive burn effect. His burn does about 30-40 hp a tic.
2. PLD/NIN or PLD/WAR work fine on him.
3. Cant be stunned or silenced or at least is highly resistant to those spells.
4. Under 20% he will Mijin. With Magus Roll, Shell 5, Barfira, and Stoneskin we still took about 1000+ damage with him at about 5-10%. He does die when he Mijinns so its best to just let him do it and pull back your dd so only your tank gets hit.