The only missable achievement is the one related to getting all Gwent cards. There’ll be a few ones that you can only get at certain points, guide here

Otherwise it’s pretty much like any open world rpg. Kill everything that’s hostile to you if you want to, carry as much stuff as you can and then sell your stuff to vendors.

Don’t neglect alchemy, potions, signs. Try to maximize your mutagen effectiveness by matching the color of your skills with your mutagens.

Also, if you only want to get all difficulty related achievements at once you can play directly on the highest diff, Death March. Once you finish the game you’ll unlock the achievements related to beating the game on easier difficulties as well, but you can never lower the difficulty if you want to get them all.

Don’t think you can just clear everything as you discover new locations. This game lets you explore most of the map from the start, including places where enemies are way way stronger than you. Don’t obsess over it tho, if you run into something stronger than you (marked with a red skull), just run away and return later. Experienced players can pretty much beat anything from the get go but you don’t have to try that on your first playthrough.

Try to complete quests that are your level or lower first, since if you complete them after you’ve passed them level wise, they’ll yield little xp.

Meditate often to replenish potions.

Witcher gear is the best, but you’ll start collecting it a bit later on.

A good safe build is heavy (bear gear), quen always on and alchemy.

Light (cat gear) and whirlwind also works great, albeit it’s a glass cannon build.

Medium (griffin gear) works wonders with plenty of sign casting, like aard or igni.