There's a lot you must change if you plan to kill those pesky High Elves!
Ok, had another go:
Characters: 1215
-Malekith = 600 (In BG)
-Supreme Sorceress w/Darkstar Cloak, Sacrificial Dagger, Pendant of Khaeleth, Null talisman and Extra level = 360 (In a warriors unit)
You cannot have both the Darkstar Cloak and Sacrificial Dagger. Drop the Darkstar Cloak and the Null Talisman for the Seal of Ghrond. Now you generate 2 extra Dispel Dice. You need the Lore of Metal as it is your 5000 point army has no way to deal with regeneration.
-Death Hag w/BSB, Cauldron of Blood and Dance of Doom = 255
There's no reason to take Dance of Doom. You already have a 4+ Ward Save. Instead take a Banner of Swiftness. The 1 inch of extra movement will make alot of difference when positioning the Cauldron.
Core: 1500
-40 Dark Elf Warriors w/Full command and Shields = 295
-40 Dark Elf Warriors w/Full command and Shields = 295
-10 Repeater Crossbowmen = 100
-10 Repeater Crossbowmen = 100
-10 Repeater Crossbowmen = 100
Build 2 squads of 20 with musician and standard. You can shoot in 2 ranks. Why wouldn't you take advantage of this? More wounds means harder to panic. More models means more ranks in melee. Its idea.
-5 Harpies = 55
-5 Harpies = 55
-Assassain w/Manbane, Cry of War and Rune of Khaine = 150 (In BG)
Assassins in melee are pretty shitty in 8th edition. You will kill a few models then die since you have no armor and only 2 wound. Convert this Assassin to a Manbane/Rending Star and toss him in with some Shades. Gives you some solid killy power against other monsters, warmachines so your Hyrdas have less opposition.
-10 Black Ark Corsairs w/Repeater Handbows = 100
Subpar when compared to Repeater Crossbowmen. Drop em for the 10 Repeater Crossbowmen that you need to run 2 squads of 20.
-20 Black Ark Corsairs w/Full command and Warbanner = 250
This is a huge waste of what should be the Sea Serpent Standard. I would also up the model count to 24 and run them 6 wide.
Special: 1401
-10 Cold one Knights w/Full command, Null talisman and Banner of Hag Graef = 360
Only 1 model in the army can carry Null Talismans. I also wouldn't bother with just 1. Try "The other Tricksters Shard, Charm Shield, and Luckstone. You can effectively disregard 2 hits and ignore pesky ward saves that will hold you back.
-10 Cold one Knights w/Full command, Null talisman and the standard of Slaughter = 360
Give this unit the Gleaning Pennant. I run 2-3 groups of 10 Cold One Knights in 3000+ and have found it nearly impossible to play them correctly and keep both in range of the BSB. I would also take the ring of Hotek. Its one of our best magic items and you dont have it.
-3 Cold one Chariots = 300
-20 Black Guard w/Full command, Kouran and Banner of Murder = 381
Very Nice
Rare: 875
-Hydra = 175
-Hydra = 175
-Hydra = 175
-Hydra = 175
-Hydra = 175
This is way over board. You have 3 Chariots and 3 Hydras. You have absolutely crap for warmachine hunting/harassment. I would drop 2 War Hydras for 2 squads of 10 Shades. Shades are way to good to ignore.
Total: 4991
How does it look now?
BG form up against the hardest looking unit in the opponents army, Hopefully with Malekith to hassle enemy shooters and armour piercing up close, with their re-rolls to hit and Blessing from the Cauldron (either to help offense or defense), Assassain to jump out for challenges they will be able to hold their own.
Big unit of Corsairs on one flank.
2 Hydras on either side of the BG.
Cold one Chariots on one flank.
Each Cold one knight unit is supported by a Hydra.
Last Hydra to fill in any gap.
Thoughts now?
PS, I already own and have painted around half of the models, and as I batch paint (to no particular level other than table worthy) Plastic models are easy to get done.
I am however using this army as an excuse to improve my metallic metal painting schemes and practice with an air brush.