In most cases, it is enough if your power saturation was high to deal with your opponent (and if you have a source, PS will grow ‘passively’). Well, and in many cases the duel can be avoided. If you are forced into a duel under unfavourable circumstances, it means that your opponent has taken good advantage of your weakness.

This isn't quite the same thing as passive progression, but there are quite a few options to increase your strength rapidly.

The first and most obvious, and dangerous, is to summon a demon every time you can. Getting up to the 100 Demonology experience is a huge deal because you can choose who to summon, making it safe to summon every time you can afford the power saturation. Hecate and Mammon are the best for passive growth since Mammon offers 2 Secret Knowledge every 2 years (he'll give you a buff alongside the SK that lasts 2 years and you can't take that option while the buff persists), and Hecate offers an option every year that will give a 25% chance each for +1 base learning, a strong 1 year buff, +5 Combat Magic, or +2 SK. Mammon can also give you Genius which is incredibly good for growth.

The second is a bit less obvious but arguably more powerful since it's less risky. If you think you're gonna be getting into mage duels commonly, when you Conduct Magical Research and are given the option to pivot into defensive magic, take it. It'll give you a decision every 10 years to cast a spell that will give you absolute magic protection for 10 years plus giving you 1 CM. It'll also let you cast it a second time on someone important to you such as your heir, giving that 1 CM again. Of course, that's a bit slow for the XP, but the absolute magic protection is incredibly powerful in duels to the point a brand new mage has a chance against a legendary mage.

There's also a lot of tertiary ways to boost yourself. Casting spells as frequently as you can (even if they don't do much for you) to get more SK, dueling weaker mages just for the CM, Scrying and potion making give you SK, and of course going to as many Grand Rites as you can, especially if you don't have Source.

If you need access to Source, the most consistent way to get it for a pre-existing character that I know of is to get the Whole of Body lifestyle trait in Learning. It'll give you a Conduct Magical Research option for "Body Enhancement." Pour everything you have into it. At the end, you'll get a big permanent buff and, most likely, Source. The tooltip says it's a 50% chance to get Source but I have yet to fail that roll so I wonder if it's incorrectly showing the base chance and it's affected by how much you put into the research process.

Well... apt observations ;)