They may look rather harmless but do not underestimate this fearsome creature. Usually avoided in mines due to the fact it has very high hitpoints, so it takes a longer time to kill, increasing the chance of being swarmed by respawns of other creatures.
After the Updates/8.5, their hp was lowered from 8500 to 5800.
Though dangerous, they aren't overly powerful so a decent blocker with decent skills will suffice to keep Lost Soul at bay. However, they can retarget a lot so any shooters from the back row should keep their guard up at all times.
You should try to trap them in somewhere, since they run at low health and can be seen in dangerous places with more monsters...