Interesting. Please forgive me if this comes off as un-educated, but, if your client admits their guilt to you, then how does this condition your approach? I'm pretty sure that I've heard, before, you can still enter a Not-Guilty Plea, which, to my understanding, simply means "prove it", but I am interested in learning more about this particular situation.
I am answering this one because it is so general. It's on three parts.
We talk a lot on this forum about legal guilt versus factual guilt. It is a critical distinction to make. Legal guilt doesn't set in until a judge says so. It is the only kind of guilt you are really concerned with.
Factual guilt is what your client and most people on the street think matters.
Consider this scenario: your client tells you he punched some one in the face during a bar fight. He is factually guilty of hitting a guy. He did it and he meant to do it.
However, he first ducked to avoid getting clocked in the jaw by this person. He didn't want to fight, although the person he ended up hitting was literally saying "wanna go?" to him prior to taking an unprovoked swing.
Now you know your client may be factually guilty but he is not legally guilty. He has both a consensual fight and a self defence argument in play. While you cannot promise a specific outcome, you can advise him a judge is unlikely to find him legally guilty and he should go to trial.
So first learn to draw that distinction. It matters because a lot of self reps plead guilty to things based on their assessment of their factual guilt. And without getting proper instructions and giving your client the appropriate legal advice he could fall into the same trap (a lot of people plead to get the hell out of there because they are scared of the whole criminal process and it seems an easy out).
So. Once you have that nailed down there's the situation where your client's instructions tell you he is not only factually guilty but a judge will almost certainly find him legally guilty as well.
In that case you can advise him to plead because an early plea will offer the certainty of a lesser sentence. It is his choice to take it but your advice will probably be to get it over with for the mitigation bonus he gets for an early plea.
Qualifier: Unless Crown is being silly and not offering a joint submission on a deal worth taking, or unless there is an unfair mandatory minimum in play. In that case may as well run the trial. Maybe a witness won't show. Maybe Crown will fumble their case. It happens.
But you are probably really asking about -
Your client has a defence he can run but he also provides you with enough detail that you know he did it, meant to do it, and if the evidence were stronger he would be convicted of it.
For example, a drug dealer whose trunk full of cocaine was subject to a warrantless search. Or a rapist who wore a mask. Or a college kid who accessed child porn on his room mate's computer.
Here is where your duty kicks in. First, if you know your client did it but the evidence is not strong enough to ensure a conviction, you advise him to go to trial and you test that evidence. The drug dealer should know the whole case rides on the voir dire prior to trial regarding the legality of the search. If the cocaine is in, he's done.
Second, for things like the ID cases, you keep certain things in mind as an officer of the court: you cannot suggest to a witness that it was not your client who did it. You can only suggest that they cannot be sure that it was.
You also cannot put forward an alibi if you know it is false. You cannot put your client on the stand if you know he is going to lie. (You cannot put any witness on the stand if you know they are going to lie.)
Your theory of the case becomes "the Crown cannot prove this beyond a reasonable doubt" as opposed to "my client did not do this". It is subtle but it is important to grasp that distinction. Your client is factually guilty. But on the weak evidence, he is not legally guilty. And that is all that matters.