The Codex gives examples of how Sovereign class Reapers have been killed by combining the fire of 3-4 dreadnaughts, aswell as jumping in close to 'knife-fight' range and hit em hard while the Reapers slowly turn to face the enemy ships. One Alliance Admiral heard the Reapers were coming and had his Dreadnaughts lined up perfectly to punch some Reapers in the face the moment they came out of the Relay, actually slowing them down and presumably damaging them until too many Reapers arrived.
Furthermore, the Reapers did not jump in through the Citadel this cycle, allowing the races a lot of time to prepare and counter, something the previous cycles didnt have.
The Proteans fought the Reapers for several centuries even though Javik clearly states their centralised and standardised nature gave them a massive disadvantage and that this cycle could do far better because of the different tactics and races and fighting styles.
Numerous Reapers have been destroyed planetside by the use of warp/nuclear bombs carried by 'prisoners'. Basically, suicide bombers.
Last but not least, this cycle has access to Reaper weapons: The Thanix cannon developed by the Turians from the wreckage of Sovereign. Their effectiveness when mounted on a oversized frigate like the Normandy SR-2 is extremely high, just 2 shots blew up the Collector heavy cruiser. That's a ship far above a frigate's weight. Infact, that same Collector cruiser took out a patrol of Turian cruisers, and the Thanix just blew through it.
This sheer effectiveness is probably the very reason why we dont see the Thanix anywhere in the game other than in the codex. Showing the Thanix in action might lessen the feeling of urgency to find a deus-ex-machina.
Sadly, all these small victories happen offscreen. Unconventional warfare and tactics would not be enough to win in the long run because of the sheer amount of Reapers. So an advantage would have to be found.
That advantage HAS ALREADY BEEN FOUND, 2,5 years before ME3, when Sovereign's avatar in the form of Saren's body was destroyed by Shepard. Suddenly Sovereign lost his shields and didnt return fire. Why was this completely hand-waved away insted of using it as a very cool and fitting way to gain the advantage over the Reapers? Some signal disruption was going on the moment Shepard destroyed Saren's body, and noone even mentioned how that seemed odd or how examing it could perhaps lead to some way to reproduce it and neuter the Reapers.
But no...sadly the idea moment Admiral Hackett contacts us for the first time, the idea of being unable to win without space-magic or last minute super-duper deus-ex-machina gets shoved down our throats. I had a major feeling of dread when Hackett's transmission mentioned
"cannot win conventionally", "go to mars archives" "only way to stop the Reapers"......