In Assamese there is an idiom that means 'striking unnecessarily hard when the opponent is already weakened'. Is there any such idiom in English that could mean the same?
There is 'kicking a man when he's down'
verb To kick a man when he's down is to attack at the persons weakest moment. It defies the gentlemanly code of ethics, and does detract from reputation. Used literally or figuratively, it still has pretty much the same meaning.
It's not an idiom, but perhaps the word overkill might work.
Overkill: Excessive use, treatment or action
Source: Oxford Dictionaries Online (http://www.oxforddictionaries.com/definition/english/overkill)
One of the usage examples on this page is
While it may seem like overkill, the military was finally satisfied.
To kick someone while they're down: Originating from the fighting ring where it's bad form to keep beating a person while they're down or out, it now also means to make things worse for someone who is going through a difficult time.
While not perfect, there's also:
Hitting below the belt: Striking out, either physically or verbally, in an unfair way.
In American English we call that taking a cheap shot (TFD):
cheap shot n
1. (in sports) a blow, shove, or tackle maliciously directed against an opponent who is defenseless or off guard.
2. any mean or unsportsmanlike remark or action, esp. one directed at a defenseless or vulnerable person.
sitting duck Fig. someone or something vulnerable to attack, physical or verbal.
(Idioms by The Free Dictionary)
A few more relevant idioms meaning to use too much force to accomplish something (not necessarily implying that one is involved in a battle or a physical fight, but may be that as well):
A similar one is to break a butterfly on the wheel
To use a cannon to kill a fly (and its variants like to kill a mosquito with a bazooka)
There is a concept in American English called "running up the score". It means to thrash the opponent after you've already won, or to embarrass them in an unsportsmanlike fashion, because you're so much more capable than they are.
For a slightly different nuance, consider adding insult to injury. This has the same basic meaning as "kicking someone when they're down," but implies a more calculated breed of overkill. Someone who kicks their opponent while they're down might only be doing it because they are so angry they haven't noticed that their opponent is no longer fighting back. Someone who adds insult to injury has coldly thought about it and decided that honor is not satisfied with injury alone.
In American football, there is a foul called "unnecessary roughness". For example, in the NFL, if the player with the ball falls down (rules are different if the player is pushed down) and is already lying on the ground, then the defensive player only has to touch him in order to tackle him. A hard hit in this situation may bring an unnecessary roughness penalty.
In common English, you could use it as a noun. "Why did you yell at him in the meeting? Don't you think that was unnecessary roughness?"
That is a legal definition that applies specifically to police. But the same concept can be, and commonly is, applied to any other person.
Flogging a dead horse (alternatively beating a dead horse, or beating a dead dog in some parts of the Anglophone world) is an idiom that means a particular request or line of conversation is already foreclosed or otherwise resolved, and any attempt to continue it is futile; or that to continue in any endeavour (physical, mental, etc.) is a waste of time as the outcome is already decided.
Weather like the kind they’re calling for this weekend here on the US Atlantic coast is a formidable opponent and the local weather guys/gals seem to love to use “one-two punch” (link to The Free Dictionary) and “double whammy” (link to Dictionary[dot]com) to describe what we might be in for.
Or maybe we could say that Joaquin, “as an extra precaution [and] beyond requirements,” is planning “to give us one (an extra kick) for good measure.” (from Collins Dictionaries)