While I am still liking the idea of the doppelganger Investigator//Slayer I do admit that is it a little hard to account for the CR 3 adjust.
So I was thinking of a Killoren Paladin//Swashbuckler
The reason for this is it allows Smite damage from Paladin and Killoren's Aspect of Destroyer as well as level to damage for any piercing one handed or light item.
Slashing Grace would open this up to any slashing one handed or light weapon too.
So with all threes focus on Cha, the Divine Grace ability boosting the all good saves with a high Cha, and of course the free Improved Critical at level 5, might seem like a good idea right?
A Fey Paladin of the Forest style character.
While all the listed above help with combat, Aspect of Nature gives them boosts to Knowledge Nature equal to her Hit Dice, and Aspect of the Hunter gains a +2 racial bonus to Perception and Stealth, as well as a +2 to Initiative. However, you need to pick which aspect you will be using each day. So if its a day you don't except much fighting, Nature or Hunter is pretty good fall backs, but with the combo of Level Twice to damage via Paladin/Swash you dont feel as gimped on damage output.