Amazing game, brings me back memories. The problem when reaching waves near the 100s, things get pretty stupid and unless you have maxed out everything, you stand little to no chance simply because you cant do much damage to the enemy.
Even with everything maxed out, because the game slowly increases the enemies HP, there will come a point where your bullets will do nothing to stop the enemy. Grenades become useless too quickly simply because they are too expensive for what they are worth and stop being useful (Except for dealing with riot shields at low HP). Even at waves 100+ grenades only do so much as tickle the enemy. A simple fix is just to make them deal damage by % of their max HP and at level 10, should just 1 shot enemies with the exception of bosses.
To reach high waves, I rushed the backpack upgrades in the main campaign and made sure to finish as many levels at 100% HP as possible then focused on trying to balance upgrades while saving for level 10 upgrades for maxed backpack. I managed to finish maxing my gear at wave 115.
Currently stuck at wave 125 simply because the game sends a massive wave with enemies able to tank 3-5 volleys of max damage spread shot blasts, each, with at least 5 huggers being able to tank an insane amount of damage for being suicide bombers and way too many enemies able to attack you in cover.