Artillery
Your best bet, by far, for intercepting the opposing force will be artillery. Pepper the incoming targets with lots and lots of small, fast projectiles while they're en route instead of sending ships to meet them.
Cold, dark projectiles, below a certain size, will be virtually undetectable. At orbital speeds, the relative velocities between the incoming ships and your projectiles will be huge, resulting in intense damage from collisions. Furthermore, you'll be able to afford lots and lots of them. You don't need to worry about relatively low accuracy if you can launch a cloud of a million projectiles at someone.
Even an egg-sized projectile will be far larger than the sorts of micrometeorites that most spacecraft are equipped to deal with. Investing in huge orbital coilguns that can rapid-fire such projectiles with good accuracy will be farm more economical than investing your delta-v in lifting lots of air, crew, and life support equipment.
Furthermore, almost all of the expense in setting up good artillery will be upfront, and you can continue to fire them the whole time your opponents are approaching. If your opponent wants to survive the barrage, they'll need to invest so much in extra fuel and armor that their fleet won't be strong enough to face even a small orbital fleet once they get close to Europa.
Intercept in orbit
For any ships which can survive the long-range artillery bombardment, the best option for the defenders will be to wait as long as possible before intercepting. The best time to intercept will be in orbit of the target moon. This is to conserve fuel and allow the defenders to field the largest possible defensive fleet.
Moving anywhere in space requires a lot of fuel, relative to the mass of the ships being moved, as well as a lot of additional structure in terms of fuel tanks and engine stages. Waiting until the last possible minute will allow the defenders to field as much defensive materiel for as little cost as possible.