General

personally i use these: all rank S cards

mech, shade, xenon ( buff duration, max and min crit damage, all stat boost, pdr set effect)

hayato, kanna, demon avenger, (min crit, boss damage, boss damage)

mercedes, beast tamer, demon slayer ( cd reduction, pdr, status resist)

I personaly have the following set (bossing in mind) :
SS Thief set - DB / Phantom / Xenon
S rank resistance set + effect - Mechanic/DA/DS(That status ress helps me personaly a lot)
S rank random set - Kanna/BT/Mercedes, No set effect, but useful for bossing.

Swap out the Aran one, DBs already have a drain skill. But if you really want it, then replace the DrK one with Aran since %hp isn't really that useful.

So I need to change something for Mercedes. These ones are the ones I have right now.

Dark Knight
Demon avenger
Hayato

Mechanic
Xenon
Eunwol

Kanna
Aran
Beast Tamer

Mechanic (buff duration)
Mercedes (decrease skill cooldown)

anything else is up to you, I use my link skill character cards because they're already high(er) level
I recommend hayato, kanna, and demon avenger

Also you'll want to sort them for class bonuses/rank bonuses:
http://www.basilmarket.com/MapleStory-Character-Card-System.html