A helpful tip for the original NES TMNT is to switch to a different turtle if you pick up a special weapon that you like. Particularly the powerful scroll weapon which makes the final technodrome level much easier to get through. The thing is that when you're slicing your way through a slog of enemies, they often drop more special weapons, and it's nearly impossible not to accidentally pick the icon up. And the new one will replace the one you already have, you can't carry two. Having multiple turtles with the scroll in the technodrome will make it infinitely easier to get through.
Although the final Shredder battle just requires learning his simple pattern. Basically jumping up on a platform and firing any projectile (even simple stars are effective) when you get an opening. The first time I beat it, I was surprised by how simple Shredder was. I spent forever figuring out the airfield and technodrome maze levels, thinking this game was impossible, and then after all that, Shredder is so simple.