You don't specify a technology level for your setting so there are many areas to cover.
The armor is light weight and indestructible, but that doesn't necessarily mean that armor smiths at any point in time know how to make armor that truly protects the entire body. If the best they can manage is a chest plate then the chest is obviously the only part protected At earlier points in history the armor will be somewhat crude and ill fitting with gaps between armor plates, exposed joints, etc. In those time periods things would be just like they were at any similar point in our own history.
But as they get better at making the armor so that it offers thorough protection:
* Jiu jitsu was originally used for attacking people in armor and it focuses on basically twisting their arms and legs until they break since the person in the armor cannot otherwise be harmed. That type of thing seems well suited to fighting people with super-armor.
* Many medieval polearms included hooks that could be used to pull an armored rider off their horse. With armor that no weapon can pierce, I would expect to see polearms that are even more specialized at this tactic. Loop or "man catcher" type pole arms to control an armored person. Large hooks to grab legs and pull them off balance so they fall. Very narrow "needle" points that can be pushed through eye slots or small gaps in segments of armor. 358
Swords would likely be narrow epee style weapons that can be threaded through gaps in armor while the armored person is being held or incapacitated. Rondel type daggers would be very useful as well since they were designed for pushing through small openings in armor. At this level of technology things like boiling water or burning oil that can cook them in their armor would also be very effective.
* As weapons technology advances, things would shift much more towards NBC weapons. Boiling water and burning oil would evolve into mustard gas and nerve toxins. You don't say whether this material conducts electricity but if it does then taser type weapons might be a good option. There might also be more emphasis on things like sonic weapons to deafen and disorient people or lasers to blind them.
It is also important to consider the impact on rest of the setting. If every house is made of this common and indestructible material than every house is effectively a castle or bunker. If cars are made out of it then every car is an armored personnel carrier.
And while it is not directly related to combat, I have to wonder what impact this material would have on society as a whole. If it is indestructible then what happens when you don't need it any more? A set of armor made of the stuff will never break, right? So (using our timeline) when it is the late medieval period and we have these advanced Maximilian armor suits being produced, what do you do with all of the leftover armor from ancient Greece and Rome? If you have an indestructible one room shack then what happens when you want to build a larger and more modern structure on that location? When your ancient ancestors built a motte and bailey castle with walls clad in this material, how do you tear it down and build your more modern tower? It may not be an issue at the time your story is set but at some point this world would need to come to terms with the fact that indestructible materials are bad for landfills. 347