I would jump up and cheer, crying out:
HA! I GOT YOU, I got you all SOOO good! You all thought your characters were invincible, huh? Right? Well, yeah, now you know. Now you see that they weren't such "hot stuff", huh? HA! MWAHAHAHAHAHA!!!
Then I would run around the gaming table, hollering and skipping in mirthful glee. Finally, exhausted from celebrating, I will fall back into my chair, let out a big sigh, and merrily ask the players, "So, everyone ready to return to 1st level? Let's roll up some new PCs!"