The LT18i is the Sony Ericsson Xperia Arc S. "Root" likely refers to gaining root access on this device. Here's what you need to know.

Sony Ericsson Xperia Arc S (LT18i) — Rooting Guide

⚠️ Important warnings first

• This is a 2011 device running Android 2.3 (Gingerbread) → upgradable to Android 4.0.4 (ICS).
• Rooting voids warranty, can brick the phone, and most guides/links are dead (XDA threads from 2012–2013).
• You need a Windows PC, USB cable, and USB debugging enabled.
• Back up your data first.

General rooting methods for LT18i

For Android 2.3.4 (Gingerbread) — easiest

• Use DooMLoRD's Easy Rooting Toolkit (zergRush exploit) — one-click script.
• Or SuperOneClick with zergRush.
• Requires: USB debugging ON, unknown sources ON.

For Android 4.0.4 (ICS) — official firmware

• Use Flashtool + a pre-rooted kernel (e.g., DooMKernel, Advanced Stock Kernel).
• Steps roughly:
  1. Unlock bootloader (via Sony's official unlock tool — note: this wipes DRM and may disable TrackID/Facebook integration).
  2. Flash a custom kernel with root + CWM recovery via Flashtool (fastboot).
  3. Flash SuperSU zip from recovery.

Alternative — no bootloader unlock

• Some ICS firmwares can be rooted via Bin4ry's root method or zergRush variants, but reliability varies by firmware build number.

What you'll need

• Flashtool (by Androxyde)
• LT18i firmware (.ftf) files
• Fastboot drivers (Sony)
• SuperSU / Magisk (older SuperSU for GB/ICS)
• A PC with Windows 7/10

Where to find current info

• XDA Developers → search "LT18i root" and "Arc S" forums (archived but still readable).
• 4pda.to (Russian forum) often has surviving mirrors for old Xperia files.

Reality check

Because the device is 13+ years old, most download links are dead. If you can tell me:

• Your current Android version / firmware build number (Settings → About phone), and
• Whether your bootloader is unlocked,

I can point you to the specific method and files that still work. Which do you have?