Sonic the Hedgehog 2
Mecha Sonic is rather simple and straightforward, though I did have dumb deaths to him. The part where he jumps while spinning though is...yeah you have no way to anticipate that unless you know his second spinning attack makes him jump, so of course I died to that.
Death Egg Robot was a bit cryptic to figure out how to hurt him (I had to look up a guide how to hurt him, and I think I got him confused for another Eggman boss where I thought you have to hit his underside or something), and his moves are difficult to avoid at first (the claw attack comes out extremely quickly, twice, and the bombs just hurt you even though they don't look like they should). Though...once you figure out how to attack his belly without touching his arms and you hug the left and right corners when he does that jumping attack and the claw shooting out attack (it can't even reach you if he's all the way left or right and you're all the way out left or right), then he shouldn't be too difficult.