Medals Dilemma

I assume that you're in the Industrial Age in Empire mode, right?

If not, try to get to it. Nobunaga Oda get his follow-up attack ability back, while Li Shimin deals splash dmg again, and so on.
Clear conquests that reward you with Literature to optimise your runs.

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I recommend getting an archer aura gen like Li Hongzhang, even if you're not aiming for an archcom team. His aura will benefit Washington and Nobunaga Oda, and they can provide support.

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In terms of equipment, there is a thread somewhere. Many gold sets provide good 2-piece bonuses, like atk up, so you could consider pairing these.

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IMO, for Unit Trials and the other side content, I recommend setting them aside, if your priority is to clear Empire (at least 150 cities) and Campaign (at least up to Industrial Age).
These require having alternatives as well as 2nd- and 3rd-rate gens from that class to fill the slots. E.g. For inf, we're looking at getting Petain, and Barbarossa, on top of upgrading Alexander, Joan of Arc, Caesar, and so on.

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IMO, the priority should be pairing equipment or powerful, one-off equipment / buying relevant aura gens (for your main composition / for support).

This is followed by the other choice (e.g. if you bought a gen, now pair an equipment set or buy a one-off powerful type, and vice verse.

Then, strong gens for the class you chose as your main composition. E.g., in a cavcom composition, maybe Saladin can join.

This should give you 8-12 gens that you can and want to use for Campaign. With the extra free gens, you should have about 12-16 for the colosseum, even in the harder difficulty, as well as some of the side content.