Sword Art Online: MD - How to Unlock Skill Slots and Stats Per Lvl

Here's a list Skill Slots and their stats or effects per Lvl. Skill Slots are extra stats that can be unlocked once the requirements are met, upgrade them to get added stats, buffs, debuffs, and other useful effects for your characters.

Skill Slots (I guess), are mainly used by game devs to buff up or re-balance characters as the game goes, so even a 4* character that is in a disadvantage could become more useful with Skill Slots.
Sword Art Online: MD - Skill Slots Info and Stats Per Lvl

Materials: Unlocking Skill Slots

   - First, you need to unlock the required Battle Skills from your character's Attribute Matrix, then upgrade them to max level if needed.
   - Collect required materials (rarity varies per item).

Shards and Crystals

   - Collect from daily or limited events. Farm them too from story mode stages, refer here for a list of stages and their guaranteed drops: Shards, Crystals and Holy Farming Locations

Common Medallions

   - Void, Fire, Wind, Earth, Water, Holy, and Dark Medallions drops from various limited events.
   - Available Medallions depends on the current featured event.

Uncommon Medallions

Knight's Medallions
   - Knight's Medallions for example can only be obtained from limited events.

Rare Medallions

SAO: MD Rare Medallions
   - Rare materials like Sword Dance Medallions, can only be obtained from Ranking Events (1st~3rd place rewards).
   - These Medallions, are the only way to unlock the Skill Slots of exclusive characters from Medal Scouts. Highly recommended that you keep them until a strong unit appears.

Essence and Candyboxes

Essence and Candyboxes
   - Rainbow Essence
   - Rainbow Candybox

All Materials Can Be Obtained From:

   - Limited Events - Quest Completion Rewards (check Missions List Section).
   - Score and Ranking Rewards (Limited Ranking Events, aim for 1st, 2nd, or 3rd to get rare materials).
   - Drops from various Limited Event Bosses, check event details for more details.

Exchange Shop

   - Available from the Menu > Exchange Shop
SAO MD: Stardust
   - Collect Stardusts from limited events, then exchange them with Skill Slot Items from the Exchange Shop. Note that limited items will refresh after the expiry date, so save your Stardusts until an item that you need appears.
   - Fore more info, refer here: Item Exchange Shop Info and FAQs

Known Skill Slot Stats and Effects

   -*Note that these upgrade caps will be lifted higher as the game goes, but only the game devs knows when.

[Character Weapon] Skills

Lvl 1 Lvl 2 Lvl 3 Lvl 4 Lvl 5 Lvl 6 Lvl 7 Lvl 8 Lvl 9 Lvl 10 Total
and Crit+
+50 +100 +150 +200 +250 +300 +350 +400 +450 +500 (+500)
and Crit+
+50 +100 +150 +200 +250 (+250)
1 3 6 8 10
3 6 8 10 12 (67)
   - *Note: some characters are capped at Lvl 5.

Other Skill Slots Per Lvl
15 Medallions
Ranking Event
15 (Lvl 1) or 30 (some Lvl 1 and most Lvl 2) 
   - *Tables above, credits to Toomuchfreetime.

Battle Skill 1/2/3 Transform

   - Overrides your character's Battle Skill with a new one.

   - Lvl 1 = Increase Damage vs Neutral enemies by 1.5x.
   - Currently Capped here.

   - Lvl 1 = Increases Attack by 12% or ??%
   - Lvl 2 = Increases Attack by 18% or 20%
   - Currently Capped here.

   - Lvl 2 = Increases Max MP by 10%.
   - Lvl 2 = Increases Max MP by 15%.
   - Currently Capped here.

Damage Up/Skill Damage Up (or vs a certain Element)

   - Lvl 1 = Increases Skill Damage by 5%.
   - Lvl 2 = Increases Skill Damage by 10%.
   - Currently Capped here.

Combo Window Up

   - Lvl 1 = Increases window to continue a combo by 3 seconds.
   - *Note: some characters are capped at Lvl 1.
   - Lvl 2 = Increases window to continue a combo by 5 seconds.
   - Currently Capped here.

Combo Damage Up

   - Lvl 1 = Increases damage for 30-hit+ combos by 5%.
   - Lvl 2 = Increases damage for 20-hit+ combos by 5%.
   - Currently Capped here.

Damage Down during Skill NEW!

   - Lvl 1 = ?????
   - Lvl 2 = Decreases damage attained during sword skills by 50%.

Guard Durability Up NEW!

   - Lvl 1 = Raises HP by 300 and boosts guard durability even more.
   - Lvl 2 = Raises HP by 600 and boosts guard durability even more.
   - Lvl 3 = Raises HP by 900 and boosts guard durability even more.
   - Lvl 4 = Raises HP by 900 and boosts guard durability even more.
   - Lvl 5 = Raises HP by 1200 and boosts guard durability even more.
   - Currently Capped here.

Battle Healing

   - Lvl 1 = 600 HP every 10 seconds.
   - Currently Capped here.

HP&MP Recovery on Enemy Kill NEW!

   - Lvl 1 = Gain 600 Hp and 5 MP on enemy kill.
   - Currently Capped here.

Emergency Heal (MP)

   - Lvl 1 = Restores 10 MP every 10 seconds during standby (when not in use).
   - Currently Capped here.

MP Recovery Ratio Up NEW!

   - Lvl 1 = Increase MP recovery ration when use normal attack (that's all it says...)
   - Currently Capped here.

MP Recovery while Charging

   - Lvl 1 = Restore 5 MP each 3 seconds while charging.
   - Currently Capped here.

MP Recovery on Parry

   - Lvl 1 = Gains 5 MP on a successful parry.
   - Currently Capped here.

Sword Skill 3 Enhance

   -*Note that all characters will eventually get their own set of Skill Slots, and all existing characters with Skill Slots might also get more slots or Lvl upgrades as the game goes.

Attack+ Buff

   - Increases damage done, some SS3s will also get a faster animation speed once learned.

Speed+ Buff

   - Increases Attack and Movement Speed, which will also affect MP gain, parry speed, dodging speed and DPS.

Accelerates During Skill

   - Gets a faster SS3 attack animation.

Reduced Charge/Cast Time

   - Reduces animation lags or casting time. Some will get reduced damages though due to re-balancing issues (I guess).

Increased Number of Hits

   - SS3 will get an increased number of hits which will generate more damage output. Additional hits depends per character.

Reduced MP Requirement

   - Decreases the MP required to cast SS3, this is will greatly affect SS3's damage output and MP cost.

Added Elemental Effect

   - SS3 will get a new element, but DPS will somewhat drop due to re-balancing, to avoid getting it too overpowered against a certain element (I guess).

Reduce Enemy Defense 

   - SS3 will get a nice support debuff.

Other SS3 Enhancements

   - and more SS3 enhancements, like Homing Bullets!

*If you have info on missing stats or effects above, feel free to share below, thanks! ^^

Related Sword Art Online: MD Tips & Guides:
Sword Art Online MD: Shards, Crystals and Holy Farming Locations
Sword Art Online: MD - Limit Breaking and Which Stats to Choose

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