No need for twrp with fastboot. Once you get A13 flashed, then get twrp installed. OR install A12 twrp, flash the A13 ROM, then Update twrp to the A13 version.
All the EU ROMs now, are designed for both fastboot and twrp flashing in one package, so there is no need to install twrp if you don't want to but it is an easier method of flashing updates as you don't need a pc.