Permadeath mode gets easier the further you are. Thanks to the resource boost, it's extremely easy to just replay a stage you're comfortable with to farm resources and out scale the enemies. Once you've beaten the second boss and start getting yellow currency the rest is very easy.

Each time you unlock a new currency, first prioritize buying health for the massive upgrade and to make farming much safer. Then, replay the stage until you get all the important upgrades you need such as further health upgrades and damage upgrades. This is how you'll out scale the enemies. Once you feel strong enough move on the next stage and repeat.

Health upgrades are nice and make permadeath much easier.
Damage upgrades are nice, because the faster you damage enemies, the faster they become weaker.
Rockets are amazing, many enemies have obvious weak spots you should try to shoot at.
Shield upgrade is really good since anytime a shot breaks your shield, it won't do any additional damage to you and it also clears out bullets on screen. It also regenerates extremely quickly and is quite overpowered.
Clear bullets ability is essentially an easy way to prevent getting hit. Get used to using it.

Before boss stages, make sure to buy every upgrade you can.


For the first boss, I'd recommend using one silver currency on health to be able to take an extra hit.

Once you unlock shields, the number of mistakes you can make without dying goes up an immense amount and makes the rest easy.