Ok, so test with 5volt arduino succeeded. I was able to connect it to Raspberri Pi Serial and receive output. This is a temperature station powered by 4 AAA batteries which is sending info to the base station via nRF24L01 and it logs some debug output to the serial port. I was able to connect my phone to the raspberry, then login using SSH and listen to the com port using "screen /dev/ttyAMA0 115200" command. Voltmeter is for checking that Raspberry's side is staying at 3.3volts. I used BSS138 FETs, exactly how in this Adafruits converter

This is the Arduino side (sorry for messed up setup - This Pro Mini is currently doing her job and I didn't want to distract her :))