Warped tortoises are normally invincible to damage, so you need a crystal chime in your inventory to cleanse them and make them vulnerable for a short time. Tortoises which are no longer invincible will be colored lighter than the ones which are still invincible.
Warped tortoises are vulnerable to bolts so using the best crossbow you possess is strongly recommended. Failing that, other ranged weapons will still be effective. As they attack with melee you should try to attack them from a distance.
Warped tortoises can offer regular good drops, making up to 110K each trip. You can quickly reach a bank by teleporting to the Grand Exchange using the spirit tree outside the dungeon.