It's difficult as hell, no doubt. But it's possible. Some stages may take a while (I would die upwards of 40 times sometimes), but you can get through them.
The bosses are just ridiculous, though. It took me 54 minutes and over 50 lives once to beat one of the bosses (Chefbot-9000). They're even worse than Megaman bosses. You have to memorize exact patterns, when you can attack, and when to run. Since you can only get hit 3 times (on the easier difficulty), and only get 3 minutes to kill them (maybe more on others, I don't remember), it's pretty damn rough. And to top it off, there's a boss marathon at one point, where they're even more difficult than previously. I never did beat the last boss. I just don't see how he is possible. Even if you could survive, the time limit isn't nearly enough.
And I played on "easy". I couldn't imagine it on Hell's Finest. Great game though, I loved every bit of it, until the very end.