check your opponents military strenght, if their technology is more advanced you can expect battleships while you are fighting with cruisers.
Also check if your opponent is in an alliance, if you are the agressor in the war, expect greater opposition since he has friends, and they will come to his aid.
Also focus on your war score, only attack the systems wich are are part of your war objective, look at your opponent war objectives and defend those points, deny him victories.
Make sure you have multiple fleets at your disposal, normally I have one doomstack and multiple fleets of average power in the neighbourhood. Your opponent will try attacking systems wich arent part of the warscore to goat you into committing your main force to defend, let him. Just focus on the warscore sytems, remember, even if he destroys those systems and land armies on those planets, he will never win those planets since they arent part of the war objectives.
Use the Attack-Reinforce meta, attack a with a doomstack, destroy all stations, start bombing the colony planet, as soon all reinforcements are destroyed (the green bar on the planet icon is gone) you can land your armies. Most of the time I build a military station near the planet in case I get an counter attack, or leave a defense fleet in the system. Then move your doomstack to your next warobjective.