Well, you’ve got a serious problem and so does your assassin. (For purposes of the question, I’ll assume it’s a he, change to the appropriate pronoun as needed.) Although he has all his training, knowledge, and experience, he’s suddenly taken a huge hit to his coordination, speed, strength, weight, and bone density. Not only that, but because of his previous training in a much larger adult body, the reach he’ll expect to have versus the reach he’ll actually have are world’s apart. Even if he was originally trained as a child, the days where this could have helped him in a physical sense are long behind him. He’s going to have to adapt to work under an entirely different rule set, all of which will leave him vulnerable to getting killed if he tries to continue in his line of work in the same manner he would have as an adult.
He can’t fight the way he used to and the best choice for him (which he’d know) is to not fight at all. Assassin’s aren’t really trained for standup, straightforward scraps anyway. If he’s intending to kill the Infobroker (I’m not even going to ask why, but infobrokers are more useful alive), then he’s going to stalk him and kill him, preferably without the infobroker seeing him or he’ll use his child stature to get close to him and prove he’s not a threat before shanking him somewhere lethal.
In a child’s body, he’ll be much more reliant on surprise and he’ll be walking the razor thin line (which he’ll know) that if he gets caught, he’s dead as opposed to when he got caught before, he only might’ve kinda been dead.
No more jumping off rooftops. No more sniper rifles. He’s going to be limited to a very small subset of guns that don’t have much recoil. It seems weird to me that he’s not carrying a knife on his person, unless he has a rather thick skull and hasn’t gotten it through his head yet that things are no longer business as usual.
A child has two major means to beating an adult: surprise and superior force of arms. They can’t take them in one on one physical combat, it doesn’t matter how skilled they are or what they’re willing to do. Your twelve year old is about four to six years away from having a body that can use those skills. So, alternate approaches are necessary.
The Infobroker is not a bad guy, which is something he’ll pick up on because an assassin should be good at reading people and social situations. The Infobroker, like most non-psychopaths when faced with a small child, doesn’t want to kill him. He’ll use that to his advantage. Your assassin can do two things, engage in a game of cat and mouse by running away and coming back later at a more opportune time or he can curl up in a small ball and start to cry. The crying is a ploy to get the Infobroker to drop his guard, once that happens, he’ll take the knife and shank him.
If neither of these solutions work, the assassin will run with the purpose of leading the Infobroker on a merry chase to a place where he can fight to his own advantage. This may be a place he knows like an alleyway with a ready supply of objects that can easily become improvised weapons. A place with lots of people so he can convince the cops to arrest the infobroker and then sneak back in to offer him a means of escape from the precinct in exchange for information.
He can’t straight up fight him, but if the infobroker has something he wants, then the body he’s in is a convenient way to convince other people to do his fighting for him. This can be anyone from random bystanders, to crooks, to convincts in lockup, to policemen on the force. He looks small and innocent. He appears to be helpless. People who would have spat on him before will help him now.
The answer to the question is: the character must use the advantages he has access to and his brain, not the traditional combat skills he never used much anyway. Though, if he’s famous, he might not have been doing his job right anyhow.
This isn’t me saying don’t do this and I know it’s not what you were after, but it’s important to recognize the limits a character is placed under and how they use those limits to prove who they are. The most interesting thing you can do with a character who is exceptionally good at their job is put them into a situation where they have to use skills that they haven’t spent a lot time cultivating. You get all their experience but also force them to deal with the world in a new and different way. Your assassin can’t just pick up where he left off. This includes many of the combat skills he’s cultivated over his years in the profession, he has to deal with life as a child and with a child’s limitations. If he remembers what it was like to fight adults as a small child, then he’ll know what he’s in for. If he doesn’t, then he better learn quickly or risk not just death, but crippling injury.