WEAPON SPOILERS!!!
Final weapon is pretty much trash, don't save for that one. I also don't like shortshield, bomb, squarestep, energy homers, ricochet, diagonal, boomeranger, or spliner.
When saving up, take Lightning Field (Bottom-Left) as priority vs most weaker/swarm enemies, an all around high-burst weapon that clears just about everything on the screen. Slow Time(middle) is good not only for dodging bullets but also for recharging your weapon. BFG(Bottom) is great because it clears a path all the way across the screen and you can stand inside it as it travels to avoid weak, fast enemies. Big Laser (second-to-last) is an all-around good weapon for taking out swarms and tough guys, like an upgraded railgun.
As a side note, bosses are really easy with the lightning field. Simply stand inside the boss and hold down left click. You'll have to finish the final boss using this + a bit more damage.
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