Eldar Counter System

Aspect Warriors devastate some units
Aspect Warrior DPS profiles are heavily weighted to destroy certain armour types.

But they are also dual purpose
They deal respectable damage against certain other armour types.

Defeating Eldar mandates playing against their counter system
By any means that you make Eldar fire at the wrong units, you will have victory in your sights.

The Eldar matchup is one of focus fire and focus denial.

You cannot simply ignore Eldar hard counter units
Eldar units scale impressively. They must be destroyed.

Eldar units are still a threat, even if they have no desirable infantry/vehicle targets. Many can destroy your economic structures.

Eldar make sacrifices to achieve soft counter capability
Eldar make careful decisions between highly specialised and effective units and less specialised soft counter units but less efficient though more flexible.

Engagements
More than any other race they are about the careful execution of engagements, having superior means to do so via the use of Fleet of Foot, Teleports and the Webway.

Counter Engage with the correct weapon
They are a glass cannon race, who must pay dearly for resilience in combat. This means that combat encounters end much quicker, with an increased importance on reaction times.

If Eldar walk light infantry into heavy bolters, for example, it would be painful for them.