I would not recommend to go for the speedrun one and the no healing one in your first run but there still are notes for what to look out for
- the achievements only include the base game and not the DLCs
- ask Viridiana for help on any 3 bosses - you can't miss her but there aren't many bosses she is available for so best to just burn through them (it's not required for 100% completion so you can just do it together with the speedrun on another run)
- get the normal ending first because you can get the other one afterwards but not vice versa (if you somehow manage to find out about the red portals... just don't complete all of them)
- once you enter the area called "Where Olive Trees Whither" make sure to talk to the npc in the first room before killing any additional bosses - there is also an herb in this area so check the herb quest part further down
- when you find Redento (guy that needs help to get across) feel free to do his quest but DON'T talk to him once he reaches "Patio of the Silent Steps"
- completing the quest line with the guy at the church's window in Albero locks you out of getting an item for Cleofas' quest (I tested it and it doesn't seem to be the case anymore but I would still recommend to play it safe) - the quest consists of giving him 3 items so just don't give him the 3rd one until you are done with Cleofas (you get asked whether you want to give them to him so don't worry)
- if you enter an area called "Resting place of the Sister" just leave - it's part of a DLC and can lock you out of base game content
- if you do want to additionally go for the speedrun achievement the things you have to watch out for are talking to the guy on the first screen of "Where Olive Trees Whither" and you have to deal with the first 3 herbs in the herb quest part further down - to get a route I just watched this speedrun and then used this map[mapgenie.io] to take notes with Paint - for this guide I prepared a slightly different route[imgur.com] to accommodate getting the other achievements but if you get the speedrun one separately just ignore everything in green (it's from memory so might not be fully accurate) - I personally did not use the attack glitch from that video
- for the Requiem Aeternum achievement where you aren't allowed to use your healing flasks in boss fights (it's fine on some but you would have to check a list for that) - it does not collide with any of the achievements and when I went for it I got past the part where you finish the speedrun in 2:12 (you have 3h for the speedrun) without getting locked out of any achievements and I did not preplan a route so you could get all achievements in one run but I personally would rather do these 2 afterwards (the note about not going further into "Resting place of the Sister" after entering is relevant for this achievement)
- there are 2 specific quests which are very easy to fail so those will be handled in more detail

required for 100% and gives achievement "Mediterranean Diet"

there are 6 bosses where before killing them you have to give Tirso in Albero an herb (it doesn't matter which herb) if you follow these instructions you don't have to worry about it at all
once you enter any of the listed areas keep in mind to grab the herb the note leads you to and give it to Tirso before beating any bosses (you can freely explore outside of that)
you can also just give him multiple herbs at once and then you are good for the next couple bosses

Mercy dreams - when you see Viridiana (can't miss) go to the right and use the teleporter to get to Albero - Bouquet of Thyme
Jondo - most bottom row of rooms at the far right - Bouquet of Rosemary
Where Olive Trees Whither - talk to the npc in the first room for a different quest - leave towards left and then enter the room at the bottom left - Incense Garlic

ignore these 3 until you are past Esdras
at any point before finishing Wall of Holy Prohibition - buy key from the shop in "Mercy Dream" > enter the door at the end of "Brotherhood of the Silent Sorrow" > "Mountains of the endless Dusk" from left Prie Dieu/Bonfire go one room to the right - Dried Clove
The Sleeping Canvases - from the first Prie Dieu/Bonfire leave the room towards the right - Sooty Garlic
Patio of the Silent Steps - keep going right until you are in the third room - Olive Seeds

two quest endings needed for achievements so have to save scum to get both in one run
ignore this section until you are mostly or fully done with the area "Mother of Mothers"

- do the first 3 steps of Redento's (the guy that has his hands tied behind his back) quest (DON'T talk to him once he reaches "Patio of the Silent Steps") [starts at "Mountains of Endless Dusk" > go left to "Brotherhood of the Silent Sorrow" > go right until you are at the end of "Wasteland of the Buried Churches"]
- go to Cleofas (in Mother of Mothers) [at the center there are 2 rooms that are 3 high on top of each other - enter the room to the bottom right of the upper one] - DON'T SELECT END SUFFERING
- there are 3 mini boss fights in the area above "Archcathedral Rooftops" which each give a mask
- give them to her

BACK UP SAVE (Crossing Souls achievement)
if you don't want to abuse this then just do one of these 2 on your first run and the other on a later one (you used to have to go for the "Rebirth" achievement for 100% completion but on the current version that's not the case anymore)
talk to Cleofas - bottom left of "Archcathedral Rooftops"
talk to Redento - right most room in "Patio of the Silent Steps"

RESTORE SAVE (Rebirth achievement)
can freely proceed with Redento's quest whenever you want now
talk to the guy at the window of the church in Albero
give Cleofas the item - bottom left of "Archcathedral Rooftops"
talk to Cleofas - church in Albero