Provide overlapping support
Any isolated formations facing an Eldar army will be destroyed. And quickly at that. The Eldar army is best utilized , IMO, to do FF based assaults with Aspect formations with large amounts of supporting fire from Guardian Warhosts.
So the best way to survive an Eldar attack is to ensure that your formations are placed to provide supporting fire to one another.
As well, don’t assume that your larger and/or stronger formations can survive isolated from support. Don’t forget that a canny Eldar player can withhold their Farsight ability till the end of the turn to attempt to stick together six consecutive activations enabling them to easily swamp a formation with an assault, supporting fire and possible crossfire and then withdraw those attacking formations to safety. Almost nothing can withstand that type of co-ordinated attack. So don’t offer the Eldar player that sort of opportunity.