For armies:

1. Fill combat width, esp the front line, and ideally the back line (back line is essential in end game)
2. Keep up on tech, including your mercs - need to rehire every few decades
3. Choose good battles - terrain matters (e.g. don't attack into the mountains), stack size matters; retreat if you're caught in a bad battle
4. Get some buffs - morale is most important early game, discipline and combat ability starts to matter more by mid game; you should focus on one or two areas to specialize (so idea groups and policies should synergize with national ideas)
5. Have good leaders - should have high army tradition, so your leaders should all be 2-3 star generals

Usually by mid game, I'm steamrolling everyone and am the biggest great power, even if I started small. Consider watching some streamers/YouTubers, many do a good job explaining things as they go.

For navies:

1. Watch battles and retreat if you start seeing red on your side - naval battles have a domino effect, so if you're seeing more red on their side, consider continuing, you might capture more than you lose
2. Morale is the most important factor here - if you're outnumbered, check individual ships and leave the low morale and damaged ships in port
3. If you're filling combat width, the easy strategy is to get a bunch of heavies - don't worry about the inland sea malace (heavies are fine in inland sea, galleys suck in deep water), if you can out-gun them, you'll probably win
4. Pick your naval doctrine carefully - if you get galley combat ability, consider going all galleys and go over force limit as needed - it's cheaper to be over force limit with galleys than at force limit with heavies
5. Naval leaders don't matter all that much, they can break a tie, but that's about it; individual ship morale is king in navies
6. It takes a long time for AI to repair boats, so popping in and out of port can be a great way to whittle down their navy and eventually win - make sure to retreat as needed to not lose boats
7. If you have naval dominance, destroy their navy entirely (occupy provinces with their boats, engage, repeat until it's dead)
8. Don't fight with light ships, they're a liability (see 2, they get morale hammered); transport ships are really hardy, so use them as a bullet sponge if you need to fill combat width

Enemy fleet size doesn't matter at all. You can defeat 100+ ship fleets if you can beat the ships that engage (like 20-30), you just need to get it to start to domino. So commit to either galleys (with galley combat naval doctrine) or heavies, and if you start to lose, retreat, repair, and reengage.

By the mid game, it's easy to absolutely dominate if you play the early game well. Your goal in the early game is to build a power base, which means:

• expand your borders a bit by taking good land - centers of trade, defendable mountain passes, etc
• reduce autonomy
• get good synergies in your idea groups
• border the countries you want to engage in the middle game
• build the right buildings

So by mid game, you should be a regional, if not global, power, and your time will be spent gobbling up land and converting wrong religion land. I usually stop playing about 1650-1700 because I've usually already accomplished my goals. Late game, the game should be pretty easy, except the 2-3 big powers you've neglected all game.

Just play as Sweden to get the hang of things 🤷‍♂️