So the sequence is:
1) Party learns of wizard and his love of hentai - Not Doomed
2) Party makes no preparation - Not Doomed
3) Party arrives - Not Doomed
4) Party fails to detect Wizard - Not Doomed
5) Wizard starts spellcombo - Doomed.
I disagree. Either the Ranger that was gone was their scout of they lacked decent scouting altogether. Not surprised Wizard won the Hide, but I AM surprised he won the Move Silently. Noticing the Wizard would at least give them some probability of success, determined by Initiative.
Definitely agree that they were dead at the perception check.
There were three ways that could go:
They roll and perceive wizard. Most things I know about stuff suggests they're dead. PCs don't always just assault humanoid NPCs they meet, so even if they'd spotted the wizard there is no reason to believe they wouldn't just call out. Since he's hostile, spell combo gets unleashed. They're dead.
They roll and fail to perceive wizard. They're dead.
DM fudges rolls so they can't perceive wizard. They're dead.
Either they were capable of perceiving the wizard or they weren't. Since they didn't have "perceiving guy" they were dead because the DM fudged their rolls OR because the DC was implausible/impossible for all present. The spell combo wasn't going to change, and their ability to get out of it wasn't going to change because as has been explained, they failed to prepare themselves.
Even if they HAD perceived the guy, there is nothing to suggest they could or
would have attacked him first in enough time to stop him from obliterating them. Just so we're clear, most DMs I've played with don't appreciate it when you just immediately attack every NPC you meet, so you're stuck holding or delaying actions while you try to see if NPCs are hostile. If the assumption is that your party is competent enough to destroy a level-appropriate encounter very quickly, then the trick to killing them is presenting them with a fight that they won't start. Make it so they aren't a party that can destroy a level-appropriate encounter and they're that much easier to kill.
They were Doomed the moment the session started. The wizard was almost guaranteed the freedom of surprise round, and he was guaranteed to be hostile and open with that nasty spell combo. The only break the players got after combat started was that he wasted an action going invisible.
Should they have prepared better? Of course. They didn't though, and so they were dead as soon as they got off the boat (Or landed from their fly spell, but we've already established they aren't prepped for things).