Hello
@SouthPaw!
It will depend on what you are fighting. Corpse Skin + Flamestrike is definitely a powerful combination. Corpse Skin will reduce the Fire Resist (and Poison) of the target by a flat 15% no matter what their current resist is at. This does stack with other debuffs like Rune Corruption and Discordance.
Evil Omen you will want to cast before Corpse Skin to increase its duration (Evil Omen will halve the value of Resisting Spells when used with a Debuff).
To determine what spell to use, you will want to look at their resists. If Energy resist is more than 15 less than fire resist, then Chain Lightning/Energy Bolt will be better damage than Corpse Skin + Flamestrike.
Evil Omen is unfortunately not worth it for damage spells, unless it is a very long cast time (like WoD).
Evil Omen will add 25% damage to the next spell and is a 0.5s cast time with FC2. So you can take the cast time of your damaging spell and divide it by 4. If that number is less than 0.5, then it is not worth it do to an Evil Omen > Damage Spell rotation. Even if that number is 0.5 or higher, you have to account for latency/lag so really it will only be beneficial to spells that are 3s or higher.
Example:
Let's say your flamestrike hits for 100 damage at the top end.
Flamestrike Only: 100 Damage @ 1.5s Cast Time
EO + Flamestrike: 125 Damage @ 2.0s Cast Time (more realistically would be 2.25s).
Over 18 Seconds:
Flamestrike: 3600 Damage
EO + Flamestrike: 3375 Damage
This trend will continue even in very long fights:
Over 144 Seconds:
Flamestrike: 28800 Damage
EO + Flamestrike: 27000 Damage
However, with a longer cast time spell like Word of Death, you will always be ahead on damage as of the second EO + WoD combo. So unless you only have time for a single Wod, it is more efficient to use EO + WoD everytime.
Wither is a great spell with a very short cast time. The downside is you need to be within 4-tiles of your enemies. Also, like other non-target AoE spells, it can cause an immediate change of aggro, so be careful when you use it in a large cluster that something else is tanking (like your pet).
The only other Damaging Magery spell I probably use regularly is Meteor Swarm, if I need to do AoE and the enemies have high Energy Resist and the situation is too dangerous to Wither.
If you go the route of Necromancy Mastery, you open up a few more options that are quite powerful. Conduit + Corpse Skin + Poison Strike is an
Extremely powerful AoE (probably the best in the game IMO).