How do I get all Achievements?
Get all the endings and see all the events, basically.
I'm having trouble with a boss
See Boss Preparation. It's okay if you get the M tier traits over the L tier, the L tier is just ideal for optional bosses. The point is, you need to learn to make items with decent traits. Make good equipment, healing items, and maybe a Lightning Quartz if you can't make an ideal N/A.
I'm concerned about time.
This game is the noobkiller of the series, but I'm going to ask you to relax. It's not as bad as that reputation would suggest.
First, you can do basically anything once. Visit every place, bonk most enemies, gather at every node. Return trips you want to plan out more, especially around license point granting things.
You can get license points through virtually anything. Make all the items in a book once, do different types of requests, do 10 requests per rank, explore entire regions, beating bosses... Some areas have special points, such as a reward for gathering at all the nodes or visiting a landmark. It's best to just check out the license menu if you're worried.
Finally, several characters are actually beneficial to do events with. To name them: Rorona, Marc, Sterk, Gerhard (Specialty Liquor), Iksel (Cooking Class). You will get recipes, a boat and carriage upgrade, and helpful items.
How do I handle character events?
For endings specifically, see Endings. You may want to avoid triggering ends until the very end of the game.
For events, it's mostly a matter of showing up at the right places as your friendship raises. I would suggest checking out the event sections for details, but a lot will be fairly organic as long as you remember to visit all the areas of both towns sometimes.
To get events, you need friendship. You raise it by putting them in the party and adventuring with them. It will max out at 60 this way, and that's exactly what you want (no higher -- requests will raise it above that cap). It's pretty easy to just shuffle your party around as they max out friendship.
Oh and Mimi wants to be Level 50 and has some other requirements. Do look at her event page.
Are there missables?
With regards to the time limit, you can run out of time to do the things you want. But in terms of time sensitive, there are two things.
The first is the Festival in Year 5. There are two issues with it. The first is that if you don't show up on the exact day you need to (it varies depending on when you trigger the event, exactly two months later), you missed it. The second is that Pamela will be out of town during certain endgame events, so it's possible to never have her return to town to complete the festival.
The second is associated with the Filly and Tiffani Events. You have to show up to Iksel's place exactly two days after a certain event.
If you care, there are missable license points too, but if you're going for the full 4000 you did that to yourself. The Chim Pie recipe also qualifies as a missable, though I strongly suggest you miss it as it makes getting the Chim End harder.
Finally, if you trigger multiple endings at once, it's possible to lock yourself out of lower priority ends.
[PC] Why does my game run like garbage?
Plug in a controller. Yes, really.
Alternatively, if you are on laptop, you need to make sure the game is not running on integrated graphics.
Windows 10 Settings -> Search "Graphics settings" -> Browse -> Select the game exe -> Options -> High Performance -> Save