Double Dragon Arcade can easily be beaten if you have infinite money, as there's no limit on how much you can pay to win. I've beaten it using probably at least 50 coins
Double Dragon NES is hard and for me it required savestates, especially the 4th stage (the first 3 are ok). In the end you need a lot of luck, because if you're a frame late to punch the enemy he punches you instead sending you to your doom.
Double Dragon for the arcade doesn't take a lot of quarters to beat, but of course most spend a lot of quarters only to find that out. LOL! Same with Double Dragon on the NES.
With Double Dragon for the arcade, kicks and elbows are your best friend. 1 kick will stun most enemies. Elbows will knock down any enemy including Abobos. They just get back up pretty quickly until they have taken enough damage. Also, you must use the environment to your advantage. If there are cliffs and holes you can knock enemies off of for a quick kill, do it. Also taking an elevated stance like the boxes at the end of stage 2 or the cliff at the end of stage 3 where you fight the Green Abobo you want to use. Enemies can't hit you until they are on the platform so just hit them as they try to jump up. Now the trap at the beginning of stage 4 is the most daunting, but if you have patience, you can get by undamaged or minimum damage.
With the NES Double Dragon, there is enough time to where you can just punch the Williams at the very beginning without killing them til you have a minimum 3 hearts preferable 4. You get the Jump Kick at 3 and Hair Pull Kick at 4. You should be able to Jump Kick the Abobo at the end off the conveyor belt. Start off Stage 2 the same way, and you should have all 7 hearts in no time. Jump Kicking will definitely keep you out of most trouble in this game. The elbow in the game is actually stronger than the arcade version, but if your timing is off, you can take damage from enemies using it. Just 4 will drop even the Green Abobo.