To prepare yourself to use all skills and be able to wear all your gear sets, you want your stats to look like this and from there you want to stick to your main specialization stat being ENE

As you hit lvl 400 and have equipped your 4th set you will be able to pick up all the skills for both ENE and STR through some stat redistribution utilizing ENE Reset Scrolls

When it comes to Set Treasure Sets for ENE DK, you will find that you have two really good set lines you can follow. 1. Vicious Set for increased defenses and survivability and 2. Qiraji for more PvP Damage Increase and Decrease. Both are solid options and are further amplified if you have the Core Pieces.

When it comes to Runes we are looking for anything that will boost our active skills we use or our properties that focus on our defensives or attacks. Try to 

This build provides solid survivability and ATK. If you follow it is as it is laid out, you should have no issues from a F2P/Low spender to High Spender playthrough.

Minimum LvL: 400-500
Optimal LvL: 600+

This is the build that wrecks Delusions in Arena lol. Seriously, a solid build that has high defense and attack.

Recommended 400-500+
Optimal level 500+