all allies achievement

[Snakedude]

so basically all i have to do is to complete any side quest ? or is there any missable quest ?

[Bird_Dog]

if you do any and all sidequests- including the DLC content - before starting the last main quest, then yes, you will get the all allies achievement.
It is however not necessary to do all sidequests.

off the top of my mind those are the ones you need to do in order to get all the help for the last battle (as I said, I'm working off the top of my mind, so I don't have the exact names of the quests):

-the quest line with Nil. take out all bandit camps and meet Nil on the mesa. There refuse to fight him
-complete the quest with the Banuk camp where the robots are not agressive
-find and kill the fugitives from the prison farm in the Jewel
-complete the main DLC quest line
-complete the "Romeo and Juliet" questline with the Carja Noble and his suicidal daughter in Bright Market
-do both quests for the weird woman in Sunfall the one about the falsely accused general and rescueing the queen and her son
-The questline about the Oseram camp Free Heap with the big woman and her cannons.
-Complete the Hunters Lodge questline
-The quest with the Nora girl Nakoa who went to Carja lands to avenge her murderd father.

I think those are all.
Please feel free to correct me if I missed one.

[Basan™]

And here are all HZD's (Horizon Zero Dawn's) quests[horizon.fandom.com] and those marked with an asterisk (i.e., *) are the ones contributing to the "Achievement" that you're after.

Best of luck with that, tbh. I've explored most of the map and did most, if not all side quests, upon my 1st playthrough and got plenty of the HZD's "achievements" without even specifically going after those.
Imho, "achievements" aren't the best of options out there to persue just to internally tell you that you're having fun playing a game (because imho they're more to prone to either "obsessive compulsive disorder" behaviours and I know to have a mild one in that regard or to simply show off to others on-line friends/acquaintances a thing which I couldn't care less about either for they either appreciate me for what I am and not for how much trophies I can flash off the bat), but to each his/her own preferences.

[Snakedude]

thanks for info