There are lots of useless planes and weapons, so i have put together a little progression guide of most cost efficient and powerful ones:
Most cost-efficient planes:
Sentinel (you start with this one) -> Nomad (cheapest 2 slot plane) -> seagull -> commando (cheapest 3 slot plane) -> Northrop -> Triplane (best plane)
Best weapons progression:
Machinegun (you start with this one) -> fire gun (good basic damage and does additional damage over time) -> rocket spreader (very good dps), i don't recommend laser as bosses seem to be immune to it -> dark energy gun ->
plasma bomb
That's just my recommendation, You can skip some weapons and planes if you fancy a little bit of grinding (it's rather easy to do Sentinel -> Commando -> Northrop)
I always used addons Money printer and Damage Amplifier, rest is rather useless, if you have everything upgraded you can switch money printer with something else.
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