This page has a ton of resources you can use:
You may have seen it already since it's a part of SF, but in case you haven't there you go. I mostly use it for item locations, recruitment requirements, supports and for the gaiden chapter requirements.
A few things off the top of my head (in no particular order):
● It may seem like an overkill, but you could consider giving the boots to a mounted unit. One of the ways to get Roy to the throne quickly is to rescue him, and with 10 movement that doesn't take long. I usually give them to Miredy because she's already an excelent unit, doesn't have trouble rushing in on her own most of the time and can really put that 10 movement to good use.
● Speaking of Roy, don't completely neglect him. His promotion time is garbage, but you still need to use him throughout the game. Since he does have 20 or so maps before promotion, I just feed him a kill or two in every level and that's usually enough. This makes him less of a liability (as he won't be dying to random siege tomes or every time you make a little mistake), and he can actually kick ass in the last 3 maps since the Sword of Seals is OP.
● Among the axelocked units, Gonzales is my favorite one. Sadly the garbage axe hit means he'll be missing a lot until you train him, and even then he'll miss more often than other trained units. Once he promotes to Berserker though, he'll kill just about anything when he does hit. Otherwise, Paladins would be the next best option for axes (though you will need to make a point of grinding the axe rank).
● Abuse the crap out of Perceval once you get him. He's most definetely not a Jeigan; his joining stats are equal or better than those of Allan and Lance at 20/5, and he also comes with better weapon ranks. His growths are bad but his bases alone are enough to make him a beast. HM bonuses make him even more of monster. You might have heard around here that Perceval is "RNG-Proof", and he really lives up to that reputation; no matter how bad are his level ups, he's still a beast thanks to his raw base stats.
● I have a bit of a different opinion on archers. Not having an Enemy Phase really hurts and more often than not their stats aren't too impressive. Use Wolt in the chapters before you get a preparations screen, but bench him as soon as you can chose your units. Dorothy barely contributes anything by the time you get her. Klein and Igrene can (probably) ORKO wyverns with their base stats, so it's ok to bring them on, say, chapter 21 as filler units (you have more room for units that you could ever use up even if you send all of your trained units, and there's a ton of wyverns so they can help). Otherwise not really worth training.
● On that note, the Nomads are actually worth using. They're still pretty much bowlocked (even upon promotion you're stuck with iron swords, so they'll mostly be using bows anyways) but they have some more utility than archers. Having the utility of a mounted unit is really significant, and even though I just said that they won't be using swords that much they still have that option if necessary. I think Shin is better due to having a better strength cap, making him more reliable towards the end of the game, but from what I've heard Sue is ok too. Also, you can really feel that 8 movement in chapter 24. I always have Shin advance faster and kill one of the dragons waiting in each room.
● Decide beforehand if you want to go to Illia or Sacae in chapters 17-20. From what I've heard Ilia is easier, but since I've gone to Sacae every time I don't know. If you're using more than 1 Flier unit Sacae is kind of rough because you only get 1 Delphi Shield. Slow units like Lilina tend to suffer too because Nomads and Myrms as very fast (I always take Lilina here but I either feed her the Speedwings or I RNG abuse her a bit. Or both). Also, chapter 20 in Sacae is like playing Kaizo Mario blind. Unless you know exactly what you are doing, you'll probably die a million times to a random Bolting/KillerBallista/Berserker/Sleep. It'd be more reasonable if you could see, but it's a stupid fog of war map so your first playthrough will be trial and error until you figure out what is where and what you need to do to keep your units from dying.
The route split is decided by whether your Nomads or your Peg Knights have more exp at the end of chapter 16/16x (if you have more exp on the nomads you'll go to Sacae).
● Don't be afraid to use the legendary weapons but do use them sparringly. I don't like using them more than 8 times each before the endgame since having all of them makes chapter 24 much easier. I wouldn't worry about breaking them because as long as you have 1 use left at the end of chapter 22 you can still play maps 23, 24 and F. If you ever come dangerously close to breaking them you can use Hammerne, but you probably won't need to (I prefer to use hammerne 1-2 times on the warp staff and 1-2 times on the Sword of Seals, depending on how much I use Roy in the final maps).
For example, don't hesitate to use Durandal on the chapter 12 boss to one-shot him (he's the first dragon you'll face). Plus, it's hilarious to watch Roy in the cutscene making a fuss over how strong the dragon was when it litterally dropped faster than a sack of potatoes a few seconds ago xD Oh, and if you don't have any Durandal users by this point you can still wreck the boss with a Wyrmslayer, but it's funnier to kill him with Durandal.
● Don't feel bad for picking up utility units but then benching them when they outlive their usefulness. For example, you'll use Clarine a lot early on because she's a mounted healer, but she has almost zero potential for the mid and lategame. Sooner or later you'll have enough healers to not need her, she's extremely frail no matter what you do, and post promotion she does almost no damage due to her poor magic stat. Just use her while you can but ditch her as soon as you need the space for a new useful unit.
● Use this in chapter 14:
I always pick up the Silver Card, Boots, Speedwings, Warp staff and Guiding Ring. The rest, only if I feel like it.
I recommend that you take 2 theives. That should be enough to cover all the items and give you a good vision range.
You can have a strong, mobile unit like Miledy clear the northeast area on her own while Roy and Sophia slowly make their way to the throne. Just watch out for the sleep staff.
● In chapter 4, Rutger will appear as an ambush spawn about 2 tiles right of the castle in turn 6 enemy phase. Anyone other than Clarine will be completely murdered by Rutger and his killing edge, so make sure than no one besides Clarine is nearby when you end turn 6. When I played this blind Rutger killed me like this and it frustrated me to no end >.<
● The reinforcements in chapter 21 can seem overwhelming, but most of them can be avoided by never placing a unit over the box areas that trigger their spawns. Look at this map:
I believe the reinforcements triggered by crossing the purple line are unavoidable, but you can deal with them. You could also warp 1 strong unit and roy from above the blue box into the throne area and end the chapter before you trigger any of the spawn points.
● Fa is worth using but by no means a must. She'd be a much better unit if she had more uses in her divine stone and if she had more movement. Because she's very sturdy she never becomes a liability, but she'll probably lag behind in every map since anyone other than Roy should have 6+ movement and she's stuck with 5. If she's still alive, you're forced to take her to chapter 24, but even if she's still lvl1 it's not a problem as all the enemy units are dragons with 5 movement, so they'll never catch her.
● Use Saul. He's the best candidate for Aureola, and he's a very good staffer. Any staff user can be a healbot, but Saul actually has the magic stat to use ranged staffs (such as Physic and Warp) effectively, on top of starting a whole rank above in staff rank than Ellen and Clarine in a game where grinding staff rank is a massive pain in the ass.
● If you either trained Fir or you have a free slot, send either Fir or Bartre in chapter 23 to recruit Karel. You honestly don't need him if you already got this far into the game, but it's a treat to watch him level up in the single level up he gets xd IIRC, he's the only unit in all of the Fire Emblem games to be guaranteed a perfect level up. He can also be a replacement Durandal user if somehow all your sword users got screwed.
That's all I can think of for now. I wish you luck and have fun :)