Have them attacked at your pleasure, not at theirs. That should put them on the back foot. Too many DMs make the mistake of 'passive' enemies: bad guys that just sit in their dungeons and wait for the PCs to barge in and kill them. Take it the other way: bring the battle to them, especially at inconvenient times.
A few suggestions:
1. Have them accosted at a bar by a 'friendly' NPC (high Cha, good social skills, opposite gender?
) who wants to join their party. A rogue is perfect. Now, this nice young man/woman is in fact an assassin/bounty hunter/opportunist who wants their lives/equipment. After they begin to take him/her for granted, let him/her go on watch, and attack them at night. Or turn rogue in a tricky battle.
2. Have them get ambushed- the enemies are prepared, they are not. An enemy wizard, hasted and invisible, with some roguish minions can do some severe damage.
3. Plain old assassins. No confidence trick lark, just a good high-level wizard who teleports in while they're asleep and blasts the hell out of them. Or the simple high-level rogue who creeps into their room at night and slits their throats in bed.
4. Hostages. Take family members/friends hostage. Have the PCs forced to meet in a 'convenient' spot, and lay down their arms. Of course, it's a trap.
5. Corrupt Officials. Have them be attacked by some watchmen for various crimes and then try to get themselves out. Or they can attack, but then they face the whole guard establishment.
Hope these ideas help. Remember, active enemies are much more dangerous than passive ones.
A few suggestions:
1. Have them accosted at a bar by a 'friendly' NPC (high Cha, good social skills, opposite gender?

2. Have them get ambushed- the enemies are prepared, they are not. An enemy wizard, hasted and invisible, with some roguish minions can do some severe damage.
3. Plain old assassins. No confidence trick lark, just a good high-level wizard who teleports in while they're asleep and blasts the hell out of them. Or the simple high-level rogue who creeps into their room at night and slits their throats in bed.
4. Hostages. Take family members/friends hostage. Have the PCs forced to meet in a 'convenient' spot, and lay down their arms. Of course, it's a trap.
5. Corrupt Officials. Have them be attacked by some watchmen for various crimes and then try to get themselves out. Or they can attack, but then they face the whole guard establishment.
Hope these ideas help. Remember, active enemies are much more dangerous than passive ones.