This thing is pure luck but I'm pretty sure the enemy order is set, it just depends on how lucky you get with the enemy AI. Some runs you can get really far and some runs not at all. The mode is in a dinky gas station and you get surrounded easily. Once you hit wave 75 things really hit the fan. It's best to drop the zombies with gas tanks so the gas tank drops in that spot, thus you can shoot the gas tank later on when a mob is around it. No item pickups disappear or get destroyed. You get two rockets to use but you seriously better save them for waves 75-100 or good luck. I heard that shooting the middle of the ceiling does the most AOE damage with the rockets (RE2 doesn't have self-damage so no worries on hurting yourself either haha). The free DLC scenarios even introduced some new enemy types that weren't even in the main game but they're in this survival mode too. Some kind of regenerating zombies (similar to the RE4 ones in a way), that only die if you deal damage fast enough, flamethrower or the spark shot are good for them.
Worth noting, the "achievement" is an in-game achievement, not a system one. You get a normal achievement / silver trophy for finishing the mode on Normal.
RE2 has loads of in game achievements and "records". I unlocked everything until I gave up on the Tofu characters for the 4th Survivor / Hunk scenarios haha. It's pretty much surviving the same level, starting from the sewers and having to make way through them and the police station in a set path all in one run. Hunk and all the various different Tofu characters have different item loadouts. One even only has Knives, but that one actually wasn't too hard for me. Knives in RE2R work like they did in REmake 1, when an enemy grabs you you can use them to get out of the grab and take no damage.