Blade Tomes are probably the very first meta-defining non-legendary weapons in the game. With the obscene effect of adding all buffs into the damage output, these weapons are pretty much weapons of mass destruction as they could go against Triangle Disadvantages in some occasions.
In general, any mage could follow a blade tome build. In this guide however, we would focus on the common archetypes and main skills that are most commonly used alongside the blade tomes.
The main idea is to take advantage of Blade Tome's buff-addition effect to its maximum potential.
Passive Skill A
One thing about Blade Tome users is that SPD is pretty much the defining stat to have. This is because when you double an enemy, all the blade tome bonuses are essentially doubled. Hence the most optimal A slot for Blade Tome are those that increases the SPD stat. However, this could only apply to mages who are speed demons, so for those who could barely double-hit anyone, boosting ATK is the second-best option.
For speed-based blade mages, the typical trinity of offensive passive Life and Death, Swift Sparrow and Fury is up for consideration, and the skill selection boils down to how bulky (or less squishy in most cases) you want your blade mage to be.
For attack-based blade mages, Death Blow is indisputably the best skill. Fire Boost are not very good for mages as they usually have sub-par health in comparison to melee classes.
Passive Skill B
Selection of B slot is again dependent on whether the tome user is speed or attack based.
For speed-based tome users, Desperation is the most popular, and is probably the most appropriate for them, as usually you will not want to take any more hits after your first on these mages. Otherwise, Breaker of the same colour will be the dominant choice (SwordBreaker on red blade tome for example).
Honorable mention goes to Quick Riposte if your blade tome user is tanky enough to definitely live a hit, although most of the time this is pretty rare.
Passive Skill C
Blade tome's main objectives is to kill thing, so it might seemed that they should go for threaten or ploy skills to further their damage output. However, I personally prefer running buffs (Hone ATK or SPD), as I found that most blade tome users are already not lacking in terms of damage. Furthermore, the way blade tomes are usually positioned before the fight are also the most optimal to run a buff skill.
If you are on a horse (flying or not) or a wyvern and you are using a movement-type specific team, the Hone/Fortify of the respective classes are probably better on most occasions.
Blade Tome users have their specials delayed by 1 turn to "balance" the buff-addition effect (which is honestly really stupid). So shorter cooldown specials are prefered over longer ones. Hence, one should look for at the very most specials with a 3 turn cooldown such as Draconic Aura. Moonbow) has the advantage of being faster by 1 turn, however there are some trade-offs. Usually you could only activate Moonbow on the second initiation, and the skill also sometimes lack the potency to go against Triangle Disadvantages.
To summarise, a 3-turn special allows most blade mages to go against the triangle, whereas 2-turn specials are more consistent. Take your pick.
Speed mages with desperation will want Ardent Sacrifice or Reciprocal Aid to have alternatives to activating Desperation. Otherwise, movement-based support skills works fine.
The most straightforward, SPD +1 for speed mages, ATK +1 for attack-based. HP+3 might be worth it if you want to slightly compensate the lack of bulk from Life and Death.
As mentioned in the previous section, a blade mage can either kill stuff by doubling or having enough attack to kill a target in 1 hit, so you either want a blade tome user to have high SPD to double, or high ATK to kill in a single blow. For the latter style, Cavalry or Flying mages are more capable in doing so due to having access to much more potent buffs.
Below are some examples of suitable blade tome users of different styles, note that this list is not meant to be exhaustive.