So it's time to post my strategy
This time, again, there isn't anything special, actually. I think the part that took the most time for everyone (and even ended up as a failure for a few of you) is collecting shells and buried treasure which is random
So I just got relatively lucky here.
For the lot, I chose Sulani's 'Pier Perfection' in Lani St. Taz, but I then made the house smaller so it only had 63 cells (so I could use the Tiny Lot advantages). I also put a rocket base and a cauldron on the sand part.
Right after starting, I chose the Spellcaster to join The Good Timers club, then he befriended Bjorn and took the leadership from him. This way, I could always use Rally the Troops right from the start (although just a bit later it became unnecessary since the spellcaster learned to make the Potion of Plentiful Needs which worked better).
Now onto the main part. Since the sims weren't allowed to help each other, I'll just describe what each of them did separately instead of explaining it day by day.
The Vampire was the easiest part and he finished his goals first. He just always stayed within the house, waiting for 'victims'. I kept adding new victims to the empty slots of the club, starting the club gathering, making the vampire do everything he could to these sims (turn, learn personality, cast hallucination), then stopping the club gathering, removing these sims from the club and adding new ones, etc., etc. Then, when enough time passed, he just invited three first bitten sims who became his offsprings and trained them one by one. When the vampire was done with all his goals, I just bought him a coffin where he could hybernate so I didn't need to pay attention to him at all anymore.
The second one to finish was the spellcaster. Also pretty straightforward: experimented a lot with a cauldron to learn potions, practice different types of magic to learn new spells. Is there really anything else to describe lol?
Please ask if there is.
The third one to finish was the alien. I started with building the rocket right from the start of the game, despite the aspiration first goals. This way, she could collect dead aliens faster, and then just do all the other logic- and handiness-related stuff while waiting for the cooldown inbetween resurrecting dead aliens. I also found a great guide (sorry, can't share it since it's in Russian) where I could find out in advance whether the mission could give me an alien, and if yes, which options should I click. If the mission wasn't unique and couldn't result in getting an alien, I just cancelled it. All the rest was also straightforward.
And finally, the one that took the longest was the mermaid. First of all, she had a Nature aspiration from the start so she could have the "Collector" trait with helps to get the rare collectibles faster. For the odd jobs, I tried to choose only the ones that aren't rabbit holes but just require to collect or to cook something (except for the frogs - in my practice runs, I never managed to complete this one). I still had to do a few rabbit holes because there just weren't enough quicker ones, but maybe in total there were like 3 rabbit holes out of 16... The fitness skill was done by itself when the mermaid was diving for buried treasure (there is a buoy near the lot I chose, so she didn't have to travel anywhere). For the seashells, I threw some silver Kava parties (the 'Party Place' lot trait lets you start with an instant bronze, so it was quick) to get 2 shells for each party. When the other 3 sims were done with their goals (it was Friday), my mermaid traveled to an empty lot on Mua Pel'am and spent two days searching for missing seashells in the sand and diving for missing Buried treasure pieces.
Again, I thank the team for this challenge! I really wanted to play in the beautiful world of Sulani, but I never (!) had a chance/reason to do it before.
