Looks like you're not very good at fighting...

Have you noticed that Asterix and Obelix can make some sort of dash move which leaves a green line behind them? You can do this to push a Roman in front of you, so you can separate him from the other Romans and beat him with a lowered risk of getting hurt.
You can order Obelix to grab a Roman and throw him towards you by pressing a button. If you jump and hit the Roman as he flies towards you, he will be insta-defeated and drop a x3 multiplier instead of a helmet.
You can also order Dogmatix to bite a Roman by pressing a button. Most Romans, when bitten, will drop their weapons and become harmless and defenseless for a few seconds. Centurions (and, I think, lions, gladiators and pirate captains) will be only briefly stunned and hold their weapons. Romans without weapons don't drop bonuses when you beat them.
If you want to finish Rome, you will have to buy the Twister combo: the last battle is extremely hard to win without it. To use a combo, buy it from

, fill the Combo bar (on the left) by quickly punching enemies, then press the right buttons in the right order (you can check it in the Pause menu -> Combos)