Fire Emblem: Path of Radiance

Use a guide if you want to get every character blind, there are a couple you'll never get without one.

Skills will disappear if you take them off a character, so make sure you want them on there before you slap them on.

Always read all the conversations at camp, you can get items and neat bits of character backstory.

Pretty minor character spoilers:
There's a fight where you only have access to Ike and Mist, so it may be worth training her for that reason (she becomes a very good healer on promotion)

Save an Occult Scroll for Ike. The other best targets for it are Paladins, Halberdiers, and Generals. You're better off selling them than using them on anyone else.

Titania is a "Oifey" archetype - a character that starts out way higher leveled than everyone else, but still has incredible growths. She can easily be your best unit for the entire game, don't be afraid to rely on her.

IF YOU ARE GOING TO IMPORT A PATH OF RADIANCE FILE INTO RADIANT DAWN, IT HAS TO BE NORMAL OR HARD MODE. Easy Mode games cannot be imported, and actually freeze Radiant Dawn if you try.

If you play the game on Easy mode first and then move on to Normal Mode, be advised that your EXP gains cut in half between the two modes, which is the key source of difficulty. So if you breezed through Easy with power-leveled dudes, Normal is a bit more challenging.

The game is so easy on the North American/European release that every single character can be made viable with enough bonus experience, so use who you like.

Rhys is the only Light Magic user, but is a terrible combat unit without a lot of luck in his levels, so don't feel the need to train him just to have access to it.

You will be given the option to deny some recruitments for an alternate reward. NEVER deny any of them, the rewards are not worth it, even if you think you won't need the character.

Forging is really useful, and once you have access to it you should try to use before each Chapter (you can only forge 1 item per chapter). At the start of the game, when you're strapped for cash, you can do pretty well for yourself just forging Iron weapons with maxed out Might, which are pretty affordable and give you a nice edge when your Strength is low. Forged Hand Axes and Javelins are especially helpful since you have a very limited supply of their high-might variants, and maxed-stat Hand Axes and Javelins are relatively cheap and durable.

Speaking of money, your cash flow is extremely sporadic. In addition to your starting cash you get a big infusion of funds twice (once at the start of the game, once towards the end), and towards the middle of the game you're basically put on salary for about four missions, which is a big help. Aside from that, your only source of funding is from selling valuables like Gems and Statue Frags, which are technically Stat up items, but the Stat they raise is relatively useless and they sell for 4000G. Most Gems are dropped by enemies after you defeat them, but some are contained in chests, and one in particular is a hidden in the game's desert level.

Regarding the desert level, its in your best interest to find a map of the mission, because there are at least two extremely valuable treasures buried in the sand (one is a Gem, the other a stat-up item). Any map you find will have minor spoilers, of course, but they don't affect the game's story in any way.

Staves are never technically equipped by the Priests and Bishops that use them, which means that any staff can be stolen by your Thief characters. In the second half of the game you can secure a pretty nice horde of Physic rods this way. Just remember that a Thief has to have more Speed than their target, and enough Strength to carry the item's weight. This obviously isn't a problem for robbing the clergy, but be mindful if you're going to try and steal other items from proper fighters.

Regarding Supports, note that a character can only have 5 total ranks of Support, which is a fact the game kind of glosses over if you aren't paying attention. So that generally means 1 A-Supports and 1 B-Support.

If you care about character development overly much, it's a good idea to get Ike and Soren to A-Support as well as Mist and Boyd, since that can have an effect on interactions in Radiant Dawn. Ike/Soren especially, since there is actually important story and character stuff gated behind that, both in PoR and Radiant Dawn. It's kind of necessary if you want to 100% Radiant Dawn.

Finally, the way that Support works is that each Support relationship has a variable number of Chapters that two characters must particulate in to increase the Rank. Certain Supports take 1 or 2 chapters per Rank, others take 10-14 missions. If you find that a relationship just isn't ranking up, you may want to check a guide to see how many chapters it takes and plan accordingly. Boyd/Mist, Boyd/Titania and Ike/Titania are among the most drawn out, with Boyd/Mist taking most of the game to complete.

Maniac Mode tips

(This mode is only available on the Japanese version, so it won't be applicable to most people)

The earlygame and lategame are the huge difficulty spikes. The only hard midgame chapter is 17, use the rest of these chapters to train your weaker units.

Wrath + Resolve is an extremely potent combo. Nephenee comes with Wrath, and Tauroneo comes with Resolve, so it's recommended to pick one of those characters to give the respective necessary scroll to. Nephenee starts out as a very weak soldier but has good growths, and Tauroneo starts out as a pretty strong general, but might need statboosters to hit his full potential. You can always use both.

Dodgetanking is unreliable outside of characters with the Earth affinity, so it's best to rely on characters with high defenses like your armor knights and cavaliers over speedsters like myrmidons.

Every enemy that isn't swordlocked or an archer in the lategame is going to have 1-2 range, making sword units mostly useless and hand axes/javelins mandatory. Archers were never good to begin with, but don't use Rolf or Shinon (outside of the earlygame), and if you use Astrid, make her use Axes.

Elincia is more of a liability than anything since the game ends if she dies, it's recommended that you don't deploy her when you get her.

Some characters that are good on Easy/Normal/Hard like Boyd are terrible now because enemies are a lot stronger, they're by no means unusable but keep this in mind when planning your teams.

Similarly, Bonus Experience is heavily cut, so strategies like waiting to dump all your exp into Marcia/Jill aren't as effective. I recommend spreading it out among the units you want to use, prioritizing ones that come underleveled.

If you get Ena, dump as many statboosters and as much bonus experience into her as you can. This is not necessary for Nasir, but it's still worth training him as much as you can.

Pick Giffca on the final map and make sure Ike is properly trained, you will need them. Tibarn is okay, but picking Naesala will make your life very difficult.