When you go into "Monster Form", it randomly picks a monster for you. Most often it is the Troll, but sometimes it could be a Gazer or a Stone (Blue) Harpy. There are several other Monster Forms as well, not just the Troll.
As far as finding Souls to Harvest, the harder the Dungeon Level is, the faster the red lights spawn. Mark some runes in different locations on levels 3 and 4. First, no one seems to go there and second, the lights most likely will re-spawn on your next go around. I have seen most people follow the same path: they recall to the Dungeon Entrance, walk in and collect their 5 lights and then recall out to come back again thru the Dungeon Entrance. Think of where there are harder monsters to kill, like the lowers levels of Deceit where Liches and Lich Lords spawn, in Hythloth where the Demons spawn, etc. One word of caution, you need to watch your timer carefully, as you do not want to Polymorph back into Normal form with a bunch of Lich Lords around you. Once, they see you turn back to your normal self, they will know it is you when polymorhed and attack you on sight, which forces you to lose concentration and turn Normal again. It is also helpful if you can carry a couple of runes with you and if you fill your Soul Harvester up and still see "lights" close by, then mark a rune before recalling back to the Hag's House, so you can grab those without walking too far.
