My standard tactic for building a 500p list:
1. choose a cheap force multiplier
HQ. Sorcerer for example.
2. choose a single Troop choice for objective. Keep it cheap and if possible take some long-range weaponry. (Scouts?)
3. a second Troop choice. your
HQ will go here and this will be the squad advancing to the mid-field objective. About 10 bodies here if you can. Quick movement if possible.
4. at this point you should be at 350 points at most. The last thing is to take something suprising. Monstorous creature of AV13 vehicle maybe? Capable of supporting your midfielders and dealing with
TEQ and tanks if possible.
5. If you don't want the part 4, take another solid Troop choice.
Remember to keep the uprgades low at this level, but each squad should have at least some special weapon/equilavent.
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An example 500p Eldar list I came up quickly (okay, I admit it. My lists are rarely that competitive

)
Farseer
- Warlock stalking him
5 Rangers
10 Dire Avengers
- Exarch w/ Shimmershield
Wraithlord
- Glaive and Bright Lance