How about this, Black Hawk Down. Where the mob charged those two snipers in the crash site. Guess what? It hit the point where they ran low on ammo and literally for each one killed another 10 charged forth. YES, there are cases where it doesn't happen but just the same there are cases where IT DOES HAPPEN. Your example, what was the situation? How well equipped were the 30 guys? How good of a defensive location? How was the enemy quiepped, and did they have to approach through a chokepoint? Lots of factors for a ground battle but in space it's all null.
People seem to be making the error of comparing war era battlecruisers to post-war era light cruisers. The two types of ships already are different in durability and firepower, but the CCS class ships are fully functional, with shields and all. Those engineers IMPROVED the systems, they did not install Forerunner tech. To quote "Only the engines are Forerunner, the rest is UNSC". The Shields exist yes, and the engineers may have improved them/tweaked them, but they are NOT Forerunner shields.
Close quarters combat happens that way, space is (supposedly) infinite, they have room to run. Numbers don't work in open spaces.
The shields Are of forerunner design.