Difficult, but not impossible
I've seen a lot of complaints that it is too easy to get killed; however I actually enjoyed the difficulty. Getting killed shows that you need to change your strategy and actually think about the level before heading into it.
Some tips for the frustrated:
- Lightning is easily the best weapon in the game, save up early to buy it asap
- Seek cash aggressively
- Slow Time & Vortex are useful weapons, but only on certain levels so hold off on buying them
- BFG & Big Laser are only really useful against bosses (IMO) so steer clear until you have plenty of cash
- Diaganals, Boomeranger, Spliner, Reverse, Bomb & Squarestep are all useless, don't buy them