Spelunky 2

This achievement will trigger naturally as you progress through the game as 50% isn't even very far BUT just for fun, I went through the journal to see what the path of least resistance is to getting this achievement, just in case there was someone out there who wasn't very good at the game, which would be crazy.

So, I've calculated it out and even with a few rares ones thrown in there, it seems you have to get to at LEAST Olmec. If you can get past Olmec, you can get a whole bunch of easy unlocks in Tide Pools that would net you this even earlier. There are 210 items in the journal, so this achievement will unlock when you've unlocked your 105th thing.

I'll list things by category here and give descriptions to the ones that aren't incredibly obvious or already have guides for them such as Black Market or Vlad's Castle.

Places - 4
Dwelling
Jungle
Volcana
Olmec's Lair

People - 20
Ana Spelunky - Unlocked at the start
Margaret Tunnel - Unlocked at the start
Colin Northward - Unlocked at the start
Roffy D. Sloth - Unlocked at the start
Alto Singh - Unlocked under Quilback on 1-4
Liz Mutton - Unlocked somewhere in Jungle
Nekka the Eagle - Unlocked in the Black Market
LISE Project - Unlocked in Volcana
Coco Von Diamonds - Unlocked in Vlad's Castle
Terra Tunnel
Hired Hand
Shopkeeper
Tun - Lady who runs the Moon Challenge
Yang
Waddler - Found in Olmec's Lair by using two bombs in the bottom right corner of the first layer.
Caveman Shopkeeper - Can be found randomly in the back layer of any normal level, not including boss levels. Its a caveman with his own shop. Buy a rock.
Van Horsing - Found on 2-1 in Volcana. You must find an open cave first that has a silver key in it and then take it to the locked door on the same level to free him.
Parsley - Found in a cave on 2-2 jungle
Parsnip - Found in a cave on 2-3 jungle
Parmesan - Found in a cave on 2-4 jungle

Beastiary - 34
Found in Dwelling-
Snake
Spider
Bat
Caveman
Skeleton
Horned Lizard
Mole
Quillback
Found in Jungle -
Mantrap
Tiki Man
Witch Doctor
Mosquito
Monkey
Hang Spider
Giant Spider
Found in Volcana -
Magmar
Robot
Fire Bug
Imp
Vampire
Vlad
Found in Olmec's Lair -
Olmec
Misc -
Scorpion - found randomly in any pot
Bee - can be found in a bee level in jungle (random spawn)
Queen Bee - on a bee level in jungle, go inside the bee hive cave to find the queen bee
Scarab - found on any dark level or in the caves on 1-4
Golden Monkey - Place a golden idol on a kali alter and it will be sacrificed and replaced with a Golden Monkey
Leprechaun - Found randomly in Volcana or Jungle, or guaranteed spawn in all levels with the "voice echoes" level feeling.
Monty - pet that is saved, make sure they are all turned on in the settings
Percy - pet that is saved, make sure they are all turned on in the settings
Poochi - pet that is saved, make sure they are all turned on in the settings
The Ghost
Cave Turkey
Rock Dog - found in Volcana

Items - 38 - A star indicates that you can find it in a shopkeepers shop, picking up a regular item adds it to the journal but an item that is equipped on your back must be worn on your back for it to count
Rope Pile*
Bomb Bag*
Bomb Box*
Paste*
Spectacles*
Climbing Gloves*
Pitcher's Mitt*
Spring Shoes*
Spike Shoes*
Compass*
Parachute*
Udjat Eye
Kapala
Hedjet
Crown
Skeleton Key*
Royal Jelly - after getting the kapala cup, sacrifice one more pet to get a royal jelly
Cape*
Vlad's Cape
Jetpack*
Telepack*
Hoverpack*
Powerpack*
Webgun*
Shotgun*
Freeze Ray*
Crossbow*
Camera*
Teleporter*
Mattock*
Boomerang*
Machete*
Hou Yi's Bow
Wooden Shield - found in jungle from a tiki man holding one
Metal Shield*
Golden Idol
Curse Pot
Cooked Turkey - bomb a turkey to get one
Four leaf clover - kill a leprechaun or get one from the moon challenge

Traps - 10
Found in dwelling -
Spikes
Arrow Trap
Totem Trap
Log trap
Found in jungle -
Spear trap
Thorny Vine
Bear Trap
Found in Volcana -
Powder box
Falling platform
Spikeball

Total - 106 items.

So you get a pass on one item. If you can make it to Tide Pool that's a potential extra place, extra character, clone gun as an item, 6 extra enemies, 3 more traps, so 12 extra journal entries.