Er-2 is required if we are service unit

No, It's not required.

if you are registered under4 excise then file NIL return. non filing may invite troubles of concealment suspect

we are registered under central excise but we are not paying any excise and availing any credits we are 100% EOU and we are in 100% service oriented unit.

as EOU registration , returns are to be filed, even NIl returns, and it should be filed within specified time limit