Certainly the scoring needs a fix but all of them are do-able even with solo to get silver or gold. Most of the time, you have to force adds spawn or headshot alot to gain more score. Headshotting major mobs like Evolved Thrall and brute easily give you 4-5x the point if you make sure to crit kill it. The more damage you do to the mob health with crit damage, the higher the score.
Algorithm: Don’t kill Geoff right away after his 2 upgrades. On the last phase of the fight, if you stay too far away from him, he will keep spawning adds after you kill all of them. This is where you get the point to reach even gold solo. Also, the reason your score wil be low on this stage is because it’s hard to get crit on robots thanks to them having crit spot BEHIND them or right in the center behind body part cover.
Void call: After you return from the shadow realm, the boss will spawn adds and drop score every time you kill all the adds.
Renegade: If you don’t kill Jailer fast, he will keep spawning adds. You can even just stand at the spawn location door and headshot each Evolved thrall coming in for points.
Archive: Data minion thingy stall
Sentinel: easy to get gold even when solo without any trick. Just get to all the loot chest that involve climbing the spike trap.
Experiment: Try to avoid buying Thumper and headshot a lot.
Sabotage: Easy gold anyway. Loads of adds.
Heliophage: Easy gold anyway. Loads of adds.