Most efficient way to farm or increase a character's SP?

Since the new skill inheritance system is out, collecting more SP on your unit has become very valuable.

I know that SP is increased by the following:

• Leveling up; +6 to +24
• Defeating an enemy unit (who is close to your level); +1 to +3
• Healing an ally unit (available for Staff Heroes only); +1
• Merging the same heroes; +40 to +120

I have a couple of units at 5*, level 40 already, and I was looking to farm SP for them. They can no longer level up, and they don't have staves.

I know that defeating a unit gives SP, but not if the unit is too far under-leveled. I am unsure of the level cap that applies to this (is it +/- 5 levels, 3 levels?)

Also, sometimes the kill gives +1 SP, other times +3 SP, I am unsure why that happens.

I'm looking to quickly defeat many enemies with a single level 40 unit using the least amount of stamina, in short:

What is the most efficient way to farm for or increase a character's SP?

I've spent a few days experimenting, and this is what I have found:

You gain SP for defeating enemies that would normally give you experience. This seems to be when the level of the enemy is 6 or less under your own.
Confirmed twice, My level 37 killed a 31 in the training tower, gained a tiny bit of exp and +3 SP, and level 40 5* Lyn killed a level 34, gained +3 SP

In the Arena, your units gain +1 SP from defeating an enemy.

Otherwise, units gain +3 SP for defeating and enemy.

Units with staves will always gain +1 SP for using the staff.

Therefore, it seems that you can go as low as the 7th Stratum(8 Stamina) in the training tower with a team of level 40s, and still gain SP each kill that they get. If the Seventh Stratum shows level 35+ (as enemies can spawn a level lower than the shown level). If the 7th goes below 35, work on the 8th Stratum(9 stamina) for guaranteed +3 SP on every kill.

If you're trying to farm SP for a level 40 5☆ character, then the only way to get SP outside of merging allies together (which would require another of that unit -- likely not an option if they are a rare character, e.g. Hector or Azura) is to get it from kills.

At launch, your options were very limited in that you could really only farm SP in Arena battles or the Training Tower. However, thanks to numerous updates, there are some great (less grindy-feeling) ways to earn SP much faster than before:

• Double SP Events

Utilizing double SP events pays off very quickly. Assuming you have a unit sweep a team within the level range to award SP, a hero can get between 18 and 30 SP from a single battle as opposed to 9 to 15.

• Sword/Lance/Axe/Tome Valor skills

This goes along with the previous bullet point. The Valor skills (Sword Valor on Gray, Lance Valor on Summer Robin, Axe Valor on Summer Tiki, G Tome Valor on Summer Elise) will allow all units wielding the corresponding weapon type to receive double SP in a battle that a hero with that skill equipped is on the same team for. This actually compounds with double SP events, which means a hero finishing off four enemies will earn 48 SP in a single map. This is by far the most effective method of grinding SP in a short amount of time.

• Tempest Trials

While the units that can benefit from these the most are limited to 8 per Trial period (the 8 focus units receive double SP and EXP when participating, as well as stat boosts that make them extremely effective), this is a very viable way to farm SP. Even if you want to farm SP for a hero who isn't a focus unit, if you attempt to get all of the rewards, the team you use for the Trials will gain a ridiculous amount of SP. In the current Tempest Trial Mini, I started using my newly-created Super Camilla+4. Despite barely having enough SP for all of her skills going into these Trials, she's now sitting at nearly 5000 SP, which is far more than she will ever need.

For posterity, the original two efficient ways to earn SP are below:

• Arena battles

Assuming the heroes you fight are also level 40, your maxed heroes will get some SP from these. As an added bonus, these would also end up netting you some Feathers to unlock the potential of other units.

• Training Tower, rank 9

This used to be the most Stamina-conservative way to farm SP aside from Arena battles (since they don't cost Stamina), since story missions take an absurd amount of Stamina to attempt at higher levels, while this takes 9. However, thanks to the addition of new game modes and mechanics, the Training Tower is not nearly as effective as it once was.

There are now more ways to increase the amount of SP gained.

Blessings and Legendary Heroes

Blessings double the amount of SP gained. This bonus always applies, even without any Legendary Hero, and even during the wrong season.

Legendary Heroes are considered blessed by default, and therefore also profit from that same bonus.

Tempest Trials+

During the first two battles of the day, all allies will receive the stat boost that is usually reserved for bonus allies (including double SP).

All double SP bonuses stack. A blessed character who kills an enemy unit during one of the first 2 Tempest Trials+ battles of the day while a Double SP Event is running, will gain 8 times as many SP as normal — that is 24 SP instead of only 3. If someone in the current party has a Valor skill equipped and is still alive, you can even gain 16 times as many SP.

It is therefore possible to gain up to 48 SP per kill, or 16 SP per heal.