You may selectively etch Cu with regards to Ni with freshly prepared aq. sol. of ammonium persulfate (20―40 wt%), with added NiSO4 (sat.), and slightly acidified with 1 drop of conc. sulfuric acid. Etching may proceed in cold solution or at room temperature, and should be not prolonged beyond necessary; say, for more than about 10―30 min. 

Thanks Carlos...

I was going through the transese etchant compatibility chart.. It was mentioned that their ammonium persulfate based copper etchant attacks nickel.. 

have you tried the suggested chemistry.?

recently i tried another copper etcahnt based on acetic acid and H2O2 . It worked well. Nickel remains intact while copper dissolves..

Please tell the concentration of acetic acid you have tried?