The easiest way imo is using the character "Wealth"
after getting basic upgrades, dont bother buying more.
You dont need the Endless basic damage upgrade, you stack more dmg with simply getting more cells, than if you spend them.
Because you deal 1% more dmg per 20 Cells.
And you can easily hit 50 to 70k cells at wave 35.
Meaning 4000% more dmg
Vs Bosses its best to have a "Gland of the Hagfish" coupled with a "Belly of the Elichu"
So that the Bosses take more damage.
And then just 4 fully stacked Brains and try to get your weapons as high as possible