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2 x 10 Warriors 360
3 x 10 Immortals 840
3 x 3 Destroyers 450
1 x 3 Tomb Spyders 165
1 x 1 Lord (orb, Destroyer body, WarSyth) 180
Thoughts and concerns I understand that there is a low amount of warriors but for this individual annihilation game there will be no capturing of objectives and with the large amount of Immortals the necron count is still relatively high at 60.
I'd throw the destroyers into a squad of 4, and a squad of 5, three squads of three gives you tons of mobility, and the tombspyders can help out, but two killpoints is better than three killpoints. Other than that, it is a highly effective list if you ask me. Very tough, very hard hitting, brings me back to 3ed/4ed when we only needed 20 warriors for ANY list.
Remember, though, that the tomb spyders are pretty easy kill points.
As for the destroyers I would give the same advice Alzer already gave.
Personally I'd probably swap the tomb spyders for more destroyers. You'll keep your units tighty packed for this mission anyhow, I suppose. If the spyders are for blocking assault then more destroyer could maybe accomplish the same (keep enemy units from assaulting), but that very much depends what you're up against.
Yeah my friend plays orks and loves to consistently deep strike and infiltrate unites into combat with me. Which is why I have the squads broken down into 3's so yes if they assault the squad I will most likely lose it that turn however I will have more fire power in order to destroy the remainder of his troops. Which is also why I have tomb spyders.