sinou:
c:\program files (x86)\arduino\hardware\tools\avr\bin../lib/gcc/avr/4.9.2/../../../../avr/bin/ar.exe: unable to rename 'core\core.a'; reason: File exists
This type of error can be caused by your antivirus software. Try TEMPORARILY disabling your antivirus for a single compilation to see if that makes the problem go away. If so, you will need to whitelist the offending file/folder/process in the antivirus software.