What u need:
an high lvl WarSmith
a TH/PW
the character that has to be levelled (from now on i will call him T.C.T.H.T.B.L lol XD)
how does it works:
1.make a party with the WS and T.C.T.H.T.B.L
2.The WS summon the big boom and goes away (just outside the area where he would take exp so the exp will go only to T.C.T.H.T.B.L lol

)
3.the TH/PW (REMEMBER THAT HE HAS TO STAY OUTSIDE THE PARTY OR HE WILL LEACH EXP

) makes a big Train (that's why he is suposed to be high lvl so the mobs wont do him much damge and the train can be very very very BIG

)
4. the TH/PW bring the train near T.C.T.H.T.B.L and the big boom and does Fake Death
5. Immediately the WS activate the Big boom skill so all the monsters will be blown Out (that's why u should choos monsters with normal or half HP modifier

)