Maddening is solely for Fire Emblem die-hard's.
Let's get into the nitty-gritty of it:
EXP gains are lowered significantly (I don't know the exact number but it's at least 50%).
Enemy's are 3-5 levels stronger than on hard.
Almost every enemy has a problematic skill now. Thieves have pass (can walk through your units), archers have poison strike (-20% HP after they initiate combat on you), mercenaries have vantage (will always attack first if they <50% HP) and so on. These skills all come with their own challenges.
Maddening mode also brings back old-school ambush spawns where enemy's move on the turn they spawn, meaning they can attack you immediately.
Enemy AI has also been adjusted.
So let's look at what all this means for you units.
The professor and his students are no longer relative power houses who can storm through armies. The stronger enemies and lower EXP evens the power levels significantly to the point where the numbers disadvantage really does work against you now.
Enemies are really fast and can double most of your units. In my maddening new game mode the only units I had who could avoid getting doubled by most enemies were Petra and Felix, two of the fastest units in the game. Almost every enemy doubling and having higher attack on top of that means tanking is now dangerous, and in some cases not even viable.
Traditionally in Fire Emblem enemy phase is considered the most important turn because it is when your units normally tear through the enemy forces. This is no longer true in maddening, player phase is for once the one that matters, you need to make them count and clear as much opposition as possible to protect yourself from counter attacks, you don't want to fight on enemy terms.
I've long said in this game that skills matter more than levels and this is true for maddening. Units like Ferdinand and Bernadetta are too slow to double enemies but become incredibly valuable when given the Swift Strikes (A-Rank Lance Skill, can double attack before being countered) and Hunters Volley (Sniper Class mastery, double attack & +1 bow range) combat arts they become incredibly valuable members of your army.
Knowing the skill tress and what to go for is essential as is knowing how to take advantage of your units crests and personal skills.
Here are, in my opinion, the important class skills you will want to obtain in maddening:
Tier zero:
Noble/Commoner - HP+5 (very easy to get, you'll probably master it before reaching level 5)
Beginner:
Myrmidon - Speed +2, Fighter - Strength + 2, Monk - Magic +2
Soldier's Defence +2 is the weakest of the 4 class skills but they do also learn the reposition combat art which is arguably the best of the 4 (the other 3 being shove, draw back & swap).
Intermediate:
Pegasus Knight (female only) - Darting Blow
Brigand - Death Blow
Archer - Hit +20
Mage - Fiendish Blow
Dark Mage - Poison Strike
Darting (Attack Speed +6), Death (Attack +6) & Fiendish (Magic Attack +6) Blows are without question the best skills in the game. They activate when the player initiates a battle (but not when they are counter attacking on enemy phase). Even with the drawback of the skills being player turn only they are still so good for what they do that they cannot be ignored, you need these.
Hit +20 is self explanatory, no one likes to miss.
Poison Strike is considered very situational but the enemy losing 20% of their HP after battle in a mode where you will often need two units to take down one enemy is so useful. In my black eagles run combining Hubert's 1-3 range Mire spell (-6 defence to enemy after attack) with Canto (move again after action) units or archers made for a very good way of clearing out dangerous enemies without committing to the reach of other enemies.
Advanced:
Nothing particularly amazing here.
Aegis (Paladin, chance to half magic/bow damage) is significantly nerfed by the abundance of poison strike archers.
Seal Defence (Wyvern Rider, -6 enemy defence after combat) is okay but does more to help weaker units get kills than anything else.
Tomebreaker (Grappler, Hit/Avoid +20 vs Tome magic) is a good skill as all weapon breaker skills tend to be but they are also situational.
Bowbreaker (Warlock, Hit/Avoid +20 vs bows) one of the better breaker skills but with how easily bow users kill mages in this game you'll ideally not want to put magic users in bow range in the first place.
Master:
The master classes don't really have amazing mastery skills and most players won't even have the time to master those classes on hard/maddening anyway. That said however there is one skill that stands above all others in the game.
War Master (Male only) - Quick Riposte.
Oh boy let's go over what this skill does. When your unit is at <50% HP and the foe initiates combat on them your unit's attack speed becomes the opponents AS+4, meaning your unit will always double attack while the enemy will only get to attack once. This is an amazing enemy phase effect. I'll have to test this myself but assuming the skills stack, a unit with both Vantage (Mercenary, always attack first if <50% HP) and Quick Riposte will always be able to double attack the enemy before they even get an attack in, this means some units can essentially become untouchable if they can kill in one round of combat.
For these reasons Quick Riposte is a contender for the absolute best skill in the entire game.
And those are just the class skills. I haven't even gotten into personal skills, crest abilities or combat arts.
I'll just do a quick rundown of semi-generic combat arts you will want:
Grounder (sword) - Effective against filers.
Swift Strikes (lance) - Grants 2 consecutive hits.
Encloser (bow) - Enemy cannot move next turn.
Hunter’s Volley (sniper mastery) - 2-3 range, 2 consecutive hits
Healing Focus (Brawling) - restor 50% of HP.
There are many good combat arts in the game and they tend to be situational. Just look at what each of your units can learn and then go from there.
Other cool abilities:
Like combat arts these are semi-generic, some units can learn them based on skill ranks.
Weapon Prowess - Everyone learns these, increases Hit/Avoid/Crit Avoid based on ability level.
Weapon-faire - Tied to class, grants +5 attack on respective weapon. Can also be learned through S+ weapon rank.
Weapon Crit +10 - Tied to class. Can also be learned through S rank on weapon.
Dexterity +4 - C rank riding skill
Movement +1 - A+ rank riding skill.
Bow/Magic Range +1/2 - Tied to class
Close Counter - Bow rank C, can counter attack with bows at 1 range.
There's lots of cool builds you can do with this game so it's worth deciding on a long term plan for each of your units.
To finish off here are units I believe the player should always seek to recruit regardless of path taken:
Petra (Black Eagles, 10 Dexterity & C in riding) - One of the best speed growths in the game, train her to be a death blow wyvern and you have a top tier unit.
Fleix (Blue Lions, 15 Speed & B+ in swords) - A beast with high strength, speed and crit. Paralogue can reward player with a very good shield.
Sylvain (Blue Lions, 25 charm & C in reason, will automatically join you if playing a female protagonist) - Very good unit, can instantly join, the Lance of Ruin is an incredibly strong relic.
Lysithea (Golden Deer, 15 magic & B in faith) - Best mage, has two crests, learns dark magic naturally (Dark Spikes at B rank), essential if you want to kill the death knight.
Lorenz (Golden Deer, 20 charm & C in reason) - Not great, you want him for his paralogue which gives you the Thyrsus staff. Give it to Lysithea so she get's +2 magic range.
Other units are optional if you want the other paralogues and such but no matter what recruit these units. Get them to B level support if you need to, anything to have them on your team.
