Because your nukes are invisible
The battle is going to be much easier if you have a good team mate. You should allow another team mate to engage the enemy to attract her or his attention, while you nuke from behind the enemy. The enemy will not find or know he is even being hit because your nukes are invisible. Run and gun, do not let your own HP fall below 50% before going out of combat to use full Cronous gold. Use the same strategy you would do with attackers, but remember to chip away at his HP. Always focus on your target and when he run away, chase and attack. Please note that you should not chase them directly.