I'm going to use excerpts from your question, where I think main issues lie. While some of it would be debatably not an issue, it may help you understand and improve, for future posts.
I know I can win since I have a three star general and 90 army tradition and 95 prestige. But what would be the best strategy?
Even telling us your rough skill level, as well as the power of your and your enemies armies, and the current situation in-game, this is still a very broad question. Strategy games like Europa Universalis 4 cater to multiple play styles, and thus, there is no direct answer in what you should do. Different players would give a completely different response, based off their own preferences, and favoured game styles. This is great for discussion - but not very good for a direct answer. Things like this get a good response on the forums, but its not what we are about, here at the Arqade.
Any advice would be greatly appreciated.
Again, depending on who you ask, we are not about giving advice. We are about giving you the direct solution to a direct problem. Strategy guides, and game wikis, work great for this kind of help.
What would you do? Hit and run? Greet them in the open field for an enormous battle of 600k men? (I think this is VERY risky). 60% of my army are mercenaries (I guess that's important) And I am at a budget surplus of 15 gold and 3400 gold total
Not exactly broad, but ironically the direct opposite can be harmful, too. Try to consider your question in a way that will apply to others, in the future. A question like "how can I defeat this army" will be a lot more helpful to future readers then "how can I defeat this army if I have X men, Y gold, and have my enemy men and A and B?"
Aftermath: I lost a couple of really big unavoidable battles (120k vs 140k) but I also won some decisive battles. My army got stranded in France while Savoy and Denmark harassed the Commonwealth. The royal navy went to the baltic and took control. After 12 years the Commonwealth sued for white peace. VICTORY!!!
This appears to be a report on what happened when you went ahead. While unrelated to the closing judgement (From what I can tell), you never want to put this sort of thing up after a resolution. If you think you could add value to the question/answer by providing your post-experience, post it as a comment, either to the question, or the answer you think works best.
If the question doesn't adapt to the format of this SE site don't hesitate to comment and I will make it more general/more detailed.
Commentary is also something you may want to steer away from. If there is an issue, we will let you know through the regular procedures. Commentary in a post can often encourage down votes or vote to closes. If this sort of thing is an issue, it will likely be edited out, rather then flagged. Users may also edit your question to steer out of the other areas, such as being too broad. Use these edits to see what improvements re-qualified your answer, for a better sense of how you could have worded it better, to begin with.
In summary, your main issue lies in asking such a large question. Always consider if there are multiple answers that could follow entirely different paths. If you should expect a direct answer, one that should not change, regardless of the person answering it, your on the right track.
I can understand your confusion, given that your question has also attracted a +10 rating. As this is a community-based site, you can very easily attract positive review, while also falling outside of our scope. We have rules on what is accepted, and what is not accepted, to provide a sense of quality. However, if enough people vote in favour of a question, it will be reconsidered. It appears, at the time of writing this answer, your question has already been reopened and answered. I would say community always wins.