The EU doesn't have any military forces or command structure. The EU wouldn't be involved. It would be down to national governments through NATO.

In honesty, European nations have very credible air assets. With a lot of stand off missiles. The Russian forces would have been hit from the air. Especially their supply depots. Remember the Russian army is mainly supplied by rail. The gauge break on the former Soviet border cuts both ways. All the supply depots on the borders where everything is transferred from rail to road, getting repeatedly hit. They wouldn't get very far.