Connect your Xperia Arc S to your PC via the USB cable.
Copy the Superuser-3.0.7-efghi-signed.zip file to the root of your phone’s SD card.
Disconnect your phone from your PC.
Extract the Fastboot.zip file and place it in the C:\ directory on your PC. You should extract a folder named Fastboot with four files inside.
Copy the ARC-DooMKernel-ICS-BETA-V04-lower-boot.img file and place it in theFastboot folder. You should have 5 files by now in the Fastboot folder.
Connect your Xperia Arc to your PC but this time around you must be in Fastboot mode. You can do this by first plugging your USB cable to your PC. Hold the Menu key of your phone then connect the other end of the USB cable to your phone.
On your PC, launch the Command Prompt from Start > All Programs > Accessories.
In the Command Prompt, enter the following commands and hit the Enter key after each command:
Important! Once the kernel has been flashed, your phone should automatically reboot itself. As soon as the Sony splash screen shows up and the blue LED lights up, press the Back button repeatedly on your phone to enter ClockworkMod Recovery.
In ClockworkMod Recovery, select Install ZIP from SD Card.
Select Choose ZIP from SD Card then browse for the downloaded Superuser-3.0.7-efghi-signed.zip file and select it. Confirm the installation by selecting Yes on the screen that follows. It may take 2-3 minutes for the installation process to complete.
Head back to the main recovery menu by pressing the Back button on your phone.
Select Reboot System Now to reboot your phone.
Complete the rooting process by installing BusyBox on your phone, as it is required by many applications to use root access properly. You can install BusyBox by downloading the app from the Google Play Store.
Launch the BusyBox Installer app on your phone and select the Allow button to grant the application root access. Select Ok when a pop-up screen shows up. Select the Installbutton at the bottom to download and install BusyBox on your phone.
Rooting via one-click method
Download the Rooting Toolkit for the Xperia 2011 ICS (DooMLoRD_v1_Xperia-2011-ICS-ROOT-emu-busybox-su.zip, MD5HASH: dc7f4a107e7d991572219404a9ca996e) fromhere and save it to your PC.
Extract the downloaded file using any extracting software and place them inside a folder.
Connect your phone to your PC via the USB cable.
Double click the runme.bat that you obtained during extraction. This will also begin the rooting process. Wait for the application to finish rooting your phone.
After a while, you should be greeted with an “All Done” message. Your phone is now rooted.