Use Odin in the Odin_3.10.0 folder to flash the XXELLA Firmware from the .tar.md5 that you downloaded. To get Odin to list the phone again, you have to open the device-manager and under the section “libusbK devices” update the driver to the “SAMSUNG Mobile USB CDC Composite Device” one. Open Odin3 v3.10.0.exe, then check the Re-partition option. In PIT select the GT-I9300.pit file in the Odin_3.10.0 folder and in AP select the XXELLA FIRMWARE file (KIES_HOME_I9300XXELLA_I9300OXAELLA_742798_REV00_user_low_ship.tar.md5). Afterwards, flash TWRP (use the lastest .tar and flash it as AP in Odin). Boot into TWRP and then go to Advanced, then Terminal, and write the following commands:

make_ext4fs dev/block/mmcblk0p3

make_ext4fs dev/block/mmcblk0p8

make_ext4fs dev/block/mmcblk0p10

make_ext4fs dev/block/mmcblk0p12

Your phone should boot now! Sadly EFS is missing now, if you have a backup restore it with TWRP.