This game on hard is pretty doable all the way until the last boss and the level right before it (5-4). I would recommend doing the boss solo to prevent confusion as you have to do a lot of micro dodging. But do level 5-4 in co-op if you can.
Now listen up this method works like a champ and you can go through the whole level without getting hit more than a few times. We call it the CREEPING METHOD
These are tips for most levels but mainly 5-4
1.) I recommend playing a few times on easy just to know where the enemies are and what to expect.
2.) I recommend playing it in coop because you have two spark bombs, which if you do this method you might need to use them on the two side by side huge tanks that come from your side.
3.) MOST IMPORTANT. When moving up through the level, you can move so slow that only a sliver of the enemy will be showing at the top of the level as you progress. For instance the feet of a walker or the treads of a tank. STOP. Just as you see a glimpse of the enemy you'll be able to destroy it without it being able to fire back. That was the key to our victory.
Thank you and if you have any questions or I was too vague on something, I'll try to clarify