Quests show Ideal PWR level as a quick way to show recommended strength your Allies should be at to handle that particular quest. You can increase the PWR level of your Allies by increasing their level, unlocking Mana Boards, equipping gears, and Memory Gem.
An efficient way to do this is to AP (yellow stamina) to do very hard quests to get exp, materials, and gears, then use SP (blue stamina) to get materials to unlock your Mana Boards.
Viewing any Ally, you can see all the 7 main stats:
STR determines the physical damage that you deal.
INT determines magical damage as well as any healing skill/spell.
CON is for physical defense.
SPR is for magical defense.
LCK increases critical hit chance. Both physical and magical attacks have a chance to crit, and a critical hit deals x2 damage, so it's also an important stat to increase damage dealt.
The type of damage that you deal is shown when you view the abilities of the Ally. Most characters only use either STR or INT as their primary source of damage stat, but Headed for the Home Village Popoi is a unique example of a hybrid character that uses both STR and INT stats for 2 different skills.
Knowing and understanding this can help make things more efficient when it comes to what stat to focus on, especially when it comes to the Mana Board.
Support Allies like healers or mages that are used in the sub-slot can focus on just INT and MP and ignore the rest. For main Allies, you'll want to increase their HP and either CON or SPR for survivability, then STR or INT and LCK for damage.
Though, it's worth mentioning that Mega Spirit Magic uses both STR and INT of the Ally using it, so increasing INT for physical damage dealers isn't a complete waste.
Sub Unit / Paired Ally
Pairing an Ally works like a backup in case your main character gets KO'ed during battle, but there are a couple of useful other features that you can take advantage of as well.
When you pair an Ally with another that has the same element, you can see a number in percentage in the upper right, as shown in the example image above. Matching an element this way only gives a boost of ST gauge increase to the Ally in the sub-slot whenever the character in the main slot attacks. It doesn't give any form of stat boost whatsoever.
Doing this means that you can more quickly build up Mega Spirit Magic during battle since you'll be able to release ST more quickly.
Additionally, most Allies have passives that give boosts to their paired Ally like increasing their STR, INT, or Elemental Damage. While matching the same element can be ideal, you can still choose your paired Ally more freely to focus on getting the right support skills.
Search for a Brother Riesz gives a STR boost that's great for any physical damage dealer. While INT is good for characters that deal magic damage. You can increase the value of the support skill by unleashing them.
Other than choosing an Ally based on their support skill, consider bringing a healer when doing hard difficulty dungeons. You can safely heal whenever you clear a room before heading to the next area so you don't have to worry about using low-level Allies for the healer role.
Heal amount is based on the character's INT, so be sure to increase them through equipment or Mana Board. The Tale Begins is a useful Memory Gem for any unit that can heal since it increases the heal amount, while Set Off for Adventure is to be equipped on the Ally that gets the healing.
Magic damage dealers like these are also great options to bring, even if you don't have them fully raised. You can bring them out temporarily to deal some burst damage and use up all their MP then switch them out.
Briarborough Faerie Boss is a great free Memory Gem that you can obtain by progressing through the story. It lets you regen the MP when the Ally is inactive so you can do it again when you reach the final wave.
Mega Spirit Magic Gauge
Mega Spirit Magic is something that you build up during battle whenever you use a Special Technique (ST). After 3 uses of STs, the gauge will be full, allowing you to use MSM to unleash a powerful elemental attack based on its level, element, and the STR+INT of the character using the MSM.
The value that you get per use of ST (which determines the final level) depends on the Memory Gem that you equip to the character using the ST. A 2-star Memory Gem always starts with only Spirit Magic Lv. 2, and a 4-star Memory Gem has a base of Lv. 4. This can be increased by +1 when you fully unleash the Memory Gem, which requires an extra 4 duplicates to do so.
The element of the MSM is determined by which element is the most used from the gauge. If all 3 elements are different, then the final element will be random(?).
Lastly, the damage of the MSM also takes the STR and INT of the character unleashing the MSM into account. Headed for the Home Village Popoi is a hybrid character that has decent STR and INT, making him great for high spirit damage. Having a more focused stat can be easier and cheaper to build for compared to balanced/hybrid, but it's not yet known if it's worth the cost.
+1 Unleash 4★ Team
This is a specific setup that you can do if you have 3 4★ Allies that are all +1 Unleashed. This is because all of them have a passive skill that gives them 100% ST at the start of the battle, allowing you to use 3 ST right at the start so you can use MSM for a huge burst of damage to clear a room. It's an effective strategy for farming the high difficulty equipment quest since most of the enemies are very tanky, or any boss quests where it's only 1 wave.
Since the game gives 1 free 4★ Benevodon Spirit Icon, you only need to get 2 duplicates of 2 different 4★ characters and use the Benevodon Spirit Icon to get a team of +1s.
If you have 6 +1 4★ Allies, you can put 3 as the main and put the other 3 on the sub slot so you can do 2 MSM pretty quickly.
If you find that you want the AI companions to behave differently during battle, you can change their Strategy around to find a playstyle that fits you.
You can do this from the unit screen and select the "Change Strategy" on the bottom right.
In here, you can change an Ally's AI behavior to either be more aggressive or more of a healer/support type playstyle. Doing this can help make auto battles much easier to do, or help make certain characters with a healing skill more reliable.
All characters start with a "Balanced" strategy, so you'll have to fine-tune your party if you have a designated healer or a main damage dealer and you want them to play the role better.
Free Quest Reset
During a quest, you can retry without needing to use any energy. You can use this as a quick reset in case any of your units die in battle, miss your ST, etc.
Using Support Ally
For all/most of the training quests, you can use "Supports" that will allow you to use an Ally from someone in your Friend List. This is a useful feature to help tackle higher difficulty quests, especially since the Ally that people set are their mains, so you will only need to have 2 properly built Allies to do the quests.
Picking a support Ally means that you will replace the 1st Ally in your Party Member 2 with your friend's Ally. After using them, you won't be able to use the same support Ally again for a while, so there is a cooldown period.
If you plan on using this, it's better to switch your main Ally of your party member 2 into the sub slot.
Be sure to set your own Allies properly in the settings by going to the "Support" section for others to use. Here, you can set an Ally for each element, as well as 1 extra Ally in the "All" section. You'll need to properly equip them with a Memory Gem and gears as well.