Fe 4 is my favorite in the series oh man I love Genealogy 

zanderlex : thanks

The final boss can be very annoying so here is strategy to help. If you lost your rings in the boss fight prior or if you died on this one that means you have no rings and will die in one hit and I warn you if your not careful it can easily happen. Even if you have rings its easy to lose them all. Now for the strategy the first phase is not as difficult. After the cut seen you want to run to the edge of the screen if you don't you will fall down a bottomless pit. Now pat yourself on the back because your one step closer to beating the final boss. After that the actual boss fight will begin like I mentioned before the first phase is not as hard it is fairly simple robotnik's mechs hands will follow you eventually slamming down and crush you which as you may already know instant death. To defect this boss you must destroy each of it's six fingers each takes three hits to destroy. If you stay in one spot and hold a spin dash when it slams down instead of crushing you  would damage which ever finger touches you. Just rinse and repeat until each finger is destroyed. After that the second phase gets much harder if you don't want to fall down a bottomless pit you must keep running to the right as robotinick's mech chases you.Even if at this point you still have rings once you get hit you will lose all your rings if not all but one. For it's attacks it will  shoot a fire ball down ward at you just jump to dodge it. To beat it hit it once they it will aim its cannon at you it is very hard to dodge before it fires hit it. To dodge the cannon your timing must be very persist if you mess up you will die and have to start over again. If your tails simply just fly to the top of the screen until it fires. It's much for difficult with sonic to dodge it listen to the charging sound. Once it stops  jump if you done this correctly you should of dodged it. Just repeat until it gets destroyed. On the third and finally phase what every you do don't let go of right. Just chase robotinick and jump into him and try not to fall off the edge.Now if you done everything correctly you would have beaten the final boss of sonic 3 and knuckles with sonic or tails. I hope this guide was helpful to you and I will soon have guide on knuckle's is final boss and dooms day zone.