Yes, flight to BKK and then limo / taxi
I may look at the otion of a train - sounds like a pain otherwise.
The flight from SIN to BKK is about 2 hrs 20, then the limo/cab ride 2 hrs 30 to Hua Hin.
Forget the train unless you want to travel for several days
The train from Singapore to Hua Hin will take several days, if you left for example on a Monday you would not get to Hua Hin until the Wednesday
Depart Singapore example Monday at 1345 arrive KL 2025
Depart KL 2300 arrive Butterworth Tuesday 0630
Depart Butterworth 1420 arrive Hua hin 0630 Wednesday
Depart Butterworth ( Malaysia ) 1420 arrive Hua Hin next day at about 6.30
I would suggest flight + cab :)
Its hardly a pain, the flight is about 2 hours, then a car can get you to HH in 2.5 hours. Compare that to a train ride - much bigger pain IMO.
You 'could' use Firefly from Changi to KL Subang and then Berjaya to Hua Hin if the connections were ok, but you would have to re-check-in at Subang for the Berjaya flight, but that is very easy indeed, but as with all connections using different airlines you take that risk
Train to Kuala Lumpor, then flight direct to HH