Get a +10 Blade[4] and slot it in. Thats what I did with my first doppel.
My Merchant, Knight, Assassin could all use it

Apart from that I could make novices upto level 60 and then change them into other classes like mages or archers and I would reset stats to get great damage.
The second card should be another Doppel followed by a Phreeoni and finaly a Turtle gen.
DDTP.
[Sin]
You will rock as a sin with this weapon.
[Knight]
You wont have fast pierce but you can always use bash and Bowling bash with your DDTP.
[Smith]
You can use overthrust with sword but it can break occassionaly so you must carry iron ores to repair it as well.