To maximize efficiency without using third-party scripts that may compromise your account, use these "legal" farming methods:
: Standard for early levels. Use a simple auto-clicker (if allowed) while attacking training logs in any village to gain XP while away from your keyboard. SHINDO LIFE NEW INF SPINNER NEW AUTOFARM
: Focus on Boss Missions (Green Scrolls) which offer the highest XP-to-time ratio. SHINDO LIFE NEW INF SPINNER NEW AUTOFARM
: Participating in limited-time event bosses like those from the 5th Anniversary provides massive RELL Coins and rare Bloodline drops. SHINDO LIFE NEW INF SPINNER NEW AUTOFARM