Dynasty Warriors 2: 100% Checklist:
Mandatory Requirements:
These are the requirements that I would consider to be actually necessary to reach full completion. Now, you may say, "well if this isn't absolutely everything, then is it really 100%?" and to that I would say, yea, technically you're correct, this part of the list alone is not absolutely everything that's in the game; it is however all of the actual somewhat substantial content, and everything that the game itself actually tracks (with 2 exceptions).
Musou Mode With All Characters:
Fairly self-explanatory, complete the main story mode of the game, Musou Mode, with every available character. Not every character has a Musou Mode campaign, in particular any of the characters not associated with the three main kingdoms are not available in Musou Mode. These other characters are only playable in Free Mode, THIS SECTION DOES NOT INCLUDE FREE MODE!!! There is no in-game tracker of any kind for which characters you've completed which stages with on Free Mode, aside from the leaderboard that tracks clear speed; however this combines clear times from both Musou Mode and Free Mode onto one leaderboard, and only keeps track of the 10 fastest times per stage.
Shu Han Campaign:
Cao Wei Campaign:
Sun Wu Campaign:
Other Characters
Unlocking All Characters:
Most characters are pretty simple to unlock, just complete Musou mode with all available characters and you'll unlock all but 1 character. The one exception to this unlock condition is Lu Bu; he usually has special difficult unlock requirements in every DW, and even this game is no exception, to unlock him you need to reach 1000 K.O.s on the map "The Battle at Hu Lao Gate". This seems a lot harder than it actually is, for a typical player an average run of Hu Lao Gate will end with around 400-600 K.O.s, just barely around half of the required count. There is a very simple trick to cheese this achievement though, enter the tunnel right before Dong Zhuo and grind the infinitely respawning soldiers. There are a few important notes to keep in mind about this method, DON'T GO FAR ENOUGH INSIDE TO ALERT DONG ZHUO, or any of his other generals for that matter, and especially not the gate captain to the right. Killing Dong Zhuo ends the mission, and killing the gate captain will stop the infinitely respawning soldiers. Your bodyguards will also constantly get in your way, stealing your kills and just blocking the path (as usual). There isn't really any easy solution to this, but what I've found is the best way of dealing with it is to occasionally step out of the tunnel into the field in front of it. Doing this will make your bodyguards also follow you out, giving enemy soldiers time to flood the tunnel instead, giving you the opportunity to kill a huge group of them before your bodyguards can get in the way again. I would also recommend doing this on normal difficulty, and increasing your character's attack stat to max beforehand. It's definitely possible to do this with base stats, and on easy/hard mode, but it's a lot harder, you want the max attack to be able to kill soldiers in as few hits as possible, and normal difficulty is the perfect balance between enemies being weak enough to still kill in just a couple of hits, while also not making the rest of your army strong enough to be able to catch up to you. One last tip, and a pretty obvious one at that, if you see an enemy drop a 2x attack item, grab it, and use your Musou attack as often as possible; the easiest way to rack up a ton of K.O.s is to kill each individual peon as quickly as possible.
Optional Achievements:
These are extra achievements that I would not personally consider to fall under 100% completion criteria. Almost all of these are not tracked by the game in any way, or will most likely be met while clearing the mandatory achievements.
Free Mode:
Each empire's Musou Mode campaign goes through 5 stages, there are 8 stages total available in the game, the extra 3 stages that are not played in Musou Mode are available in Free Mode for all characters. There are also 5 characters who do not have Musou Mode campaigns, and thus can only be played in this mode. Full completion of this mode would mean clearing all stages that are not already cleared in Musou Mode with every character.
Maxing All Characters Stats:
Each character's stats can be increased through methods involving grinding combos on generals that would take far too much explanation to include in this checklist, but here's a pretty great video guide: Dynasty Warriors 2 Grinding Guide. Each character has an Attack, Defense, and Life stat to max out, maxing these out can be incredibly helpful for completing the rest of the game, and is near-required for clearing all stages on hard difficulty, but is also an extremely tedious and long grind, and is thus not recommended if you are only planning to clear Musou Mode on any difficulty.
Top Every Leaderboard:
There are in-game leaderboards for clear times, K.O. counts, and points scored on each stage, these leaderboards are populated by non-player scores when initially starting the game. The goal for this is to reach the top of every leaderboard on every stage with at least 1 character; there is no point in trying to reach the top with every character (unless you just really want to do that for whatever reason) because the game only keeps track of the top 10 scores at a time, therefore it would not be able to keep track of scores with all 29 characters at once.
Clear All Stages on Hard Difficulty:
Entirely self-explanatory, complete every stage, with every character on the highest difficulty. The game does not track anywhere what difficulty you complete stages on, this would be entirely an honor-system type of thing to achieve, as you would have no way of proving you had done so. If you do decide to do this however, I would recommend doing it on your first run with each character, and I would also very highly recommend maxing out the stats of all characters before attempting this, hard mode is incredibly difficult and if you are going in with base stats you are going to waste a lot more time than necessary on each stage, and you are likely going to have to retry maps a lot.
General Tips:
Use a simple 2/3-hit light attack combo on pretty much everything: Using a 4-hit combo, or ANY strong attacks will knock down enemies, this isn't much of an issue for normal enemy troops, it gives them some i-frames, but for enemy generals knocking them down can make this game far more tedious and difficult than necessary. Knocking generals down gives them a chance of receiving a buff, that could be a 2x increase in attack or defense, or it can be a 50, 100, 200, or full health restoration. Defeating generals isn't generally too difficult, but if you continuously let them fall to the ground you can potentially put them in an infinite cycle of regeneration.
Kill higher ranking enemies frequently: This works two-fold, not only are higher ranking troops usually leading weaker peons, meaning if you defeat the leader there's a chance the weaker troops will run away (which means if you're trying to get Lu Bu on Hu Lao Gate, kills the weakest enemies first), but they also have a chance of dropping health restoring items, or buffs. Outside of the occasional pots and crates scattered throughout maps, which are usually placed away from where the action is happening, these healing drops are your only way to maintain high HP in the middle of combat.
Work your way up in difficulty: By far one of the biggest pieces of advice I can give someone starting DW2 is to go into settings and switch to easy mode immediately after you open the game. After doing several runs and learning the best tech for getting through a run, easy mode becomes mind-numbingly easy, however for a first time player, so much of this game's mechanics work completely counter to modern conventional genre knowledge, and I think it's best to understand the core strategy to combat before needing to also focus on your strategy for the battle as a whole. On easy difficulty enemies hit way softer, but most importantly it is pretty much impossible for your troops to get pushed back; this is the biggest factor that makes easy difficulty so boring for an experienced player, but I personally think it's perfect for someone playing for the first time.