You Can't Do The Important Part
Cast Dispel Magic using the silent and still spell metamagic feats.
One of the feats can come from a metamagic rod. The dispel check needs
to beat DC16.
Wizards, Sorcerers, Clerics, and Druids can all do that fairly easily. Beguilers can, but only if they have Dispel Magic in a Runestaff (so they can cast it and apply metamagic), or if they take the Extra Spell feat to get the ability to cast it. Bards need a way to use both metamagic feats without raising the spell level.
Buff a target without their knowledge. If True Strike weren't limited
to Range: Personal, that would have been ideal. A reverse Unluck where
the target rolls twice and picks the better roll would also work.
This is really tricky. The problem is that buff spells typically allow saving throws, and if you make a saving throw you know someone is trying to cast on you:
A creature that successfully saves against a spell that has no obvious
physical effects feels a hostile force or a tingle, but cannot deduce
the exact nature of the attack. Likewise, if a creature’s saving throw
succeeds against a targeted spell you sense that the spell has failed.
You do not sense when creatures succeed on saves against effect and
area spells.
So, that's somewhat of a problem. PCs are likely to take that saving throw if unknown spell effects are hitting them by surprise.
Have high bonuses to social skills (Bluff and Sense Motive in
particular)
Beguiler excels at this. Bard is also a logical choice. Both of them have Glibness for +30 bluff, though a Bard trying to use that and dispel magic will need bonus spells to use both at 8th level.
This Plan Sounds Overly Complicated
What troubles me about this plan is that it requires so many things to come together in order to work. I don't know your players, but IMO it runs the risk of being perceived as DM fiat making things happen if it does all work, and it's pretty likely to fail due to the number of points of failure involved:
- PC makes their buff saving throw and notices something's up
- Dispel doesn't affect the weapon, as it's not guaranteed at 8th level
- PC shooting the pistol doesn't succeed on the attack roll
- The target survives. The pistol has to do enough damage to drop them to -10 to kill them, and people are going to notice that Merciful is suppressed pretty quickly when shots start landing.
I'd recommend you consider carefully before proceeding with that kind of plot.
Alternatives
If the goal is simply to trick the PCs into shooting someone with a pistol and killing them without intending to, a simpler alternative might be to use Magic Aura and just lie about the effect on the pistol. Does the PC own this weapon, or are they being handed it by someone else for use in a duel? That's what it sounds like to me, and it'd be easy to make a pistol that doesn't actually have Merciful on it but shows up as magic. Short of casting Identify on it before shooting, the PCs wouldn't know the difference until after they shoot someone.
Even simpler is to pass them false information (via Bluff and Forgery) that the target is actually a wanted man with a bounty on his head, and see if they try and collect.
