What's the simplest way to etch Nickel and Chromium sputtered layers?
Nickel can be etched using diluted (1:1 V) $HCl$. 50 nm will be dissolved within seconds. 

The etching solution for the chromium layer contains $\ce{Ce(NH4)2(NO3)6}$+$\ce{CH3COOH}$+$\ce{H2O}$
in ratio 165g : 45 ml (con.) :100 ml DI water. The chromium layer is dissolved within 5 minutes of etching.