Place X-Bow on your side near the bridge so it shoots the enemy tower. Protect it with Tesla, Knight and Archers, and keep enemy troops from reaching it. On defense, place it in front of your tower to shoot enemy troops from range.
Tips
Place X-Bow after your opponent spends elixir to catch them unprepared.
Protect X-Bow from ground troops with Tesla and Knight.
Using X-Bow on defense stops your opponent's push and turns into a counterattack.