The main strat:
1. Spawn chance has no cap and is the best stat. Focus it.
2. Rarity is the priority (I think - not 100% sure). Max out rarity every time it shows up. If you want to get a really high score, you have to constantly reset until you get early rarity (I try to keep rarity above my level)
3. Focus ranged damage over melee. It is mathematically better than melee damage.
High score exclusive strats - risky and not good in regular runs
1. Maxxing out spawn time is very powerful, BUT once you do so spawn chance upgrades become much less common. Therefore, only ever get spawn time upgrades that also give spawn chance
2. Never get any upgrade that reduces attack delay except for attack delay down upgrades. Fervor, double attack speed, and hot drop become useless once you max attack delay upgrades.