To decide on the best rotation anywhere you have to know your spell cooldowns and durations, the creature's elemental weaknesses and strengths to different elements, and the amount of damage each spell can do.
Flimsy Lost Souls are weak to 
holy, an element sorcerers can't use, neutral to 
fire and 
ice, and with different degrees of resistance to other elements. Finally, these creatures will only receive 80% of the damage dealt with 
energy spells.
The ideal rotation should be to initiate with Expose Weakness, followed by Energy Wave, then Great Fire Wave, an AOE rune (in this case either GFB or Avalanche), Great Fire Wave again and start over with Energy Wave. Whenever the effect of Expose Weakness wears off, or whenever new creatures appear in your area of attack and Expose Weakness is off cooldown, it should be re-cast.
or
Cast Expose Weakness on cooldown, or as needed.
Cast Hell’s Core only to finish off creatures.
Do not use the Great Energy Beam unless you can hit all visible creatures with it.
Expose Weakness: This spell has a 12 second cooldown and a 16 second duration. I choose to cast the spell again as soon as it is available again, rather than waiting for it to wear off . This is because given that sorcerers run around when hunting alone, it is likely that by turn the time its cooldown has worn off, you will have in your range creatures which were not there when you first casted the spell, or perhaps you missed a few the first time, so re-casting means that those new creatures will now also be impacted by the spell.
Energy Wave: This spell has a 8 second cooldown. While Flimsy Lost Souls have some resistance to energy, this wave is so powerful that it will still deal much higher damage than a GFB or avalanche. When tested on characters of different levels and skills, an Energy Wave can do almost twice as much damage as a GFB on Flimsy Lost Soul. I choose to cast it as soon as possible because it has the longest cooldown between the two waves I will use.
Great Fire Wave: This spell is slightly less powerful than the Energy Wave, but because of the slight energy resistance of these creatures, the Great Fire Wave will deal almost as much damage as the Energy Wave, so as long as you can hit several creatures with it, it should be used as often as possible.
Hell’s Core: This is a very powerful spell, with a great area of attack. However, casting it puts the sorcerer on cooldown for all attacks for 4 seconds (2 turns). Because of it, it is best used to finish off a group of creatures before moving on to the next group.
Great Energy Beam: While this spell is very powerful, even on monsters which some resistance to energy, the area of effect is not wide enough for area-hunting. As an example, a level 500 sorcerer with ML 100 has 8 creatures on screen somewhat scattered and can choose to either hit some of them (3-4) with the beam, or all of them with a GFB. Hitting all 8 with the GFB would be equivalent to 3200 total damage (average hit at that level and ML on GFB is ~400). Hitting 4-5 (quite hard align that many) would still only do an average damage of 2440-3050 damage. This spell should be reserved for specific situations such as all visible creatures being aligned, or the last creature running away.
Sources:
- Spells cooldowns and duration: Tibia client's Spell List
- Creature weaknesses: Tibia client's bestiary
- Spells damage: Testing in game & Tibia-Stats damage calculator
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