40k wise, yeah.
Dont let them EVER get into combat.
I know it may seem simple enough but you may think 'ah that squad has been reduced to 3 models, that shouldnt be much of a threat, ill start working on the next target etc' then think again.
Deal with anything that can shoot or move really fast then outmaneuver as much as you can.
2 Bloodletters and 3 Daemonnettes (what was left out of their squads) charged my Grey Hunters and Minced an entire squad of 10 without breaking a sweat.
In theory it should be easy to beat them if you have a mobile army. Tau for example should rarely have too much trouble if you can simply outrange them or obliterate everything that comes near. If they do get into combat then its Firebreak time. Accept that you've lost the unit and pull back any units that arent in combat so they cant consolidate into you and much your entire army.