Noticed deathmax had uploaded a datamine on this without telling me, so I've looked into it a little.

Level 10 Elgif is mostly the same as regular Elgif with better numbers, but there's some extra notes to be had.

Stats

HP: 3800000 (1600000 more than level 7)
ATK: 6789 (~25% higher than level 7)
DEF: 2500

Resistances:

• Immune to Curse, Injury, Paralysis and Sickness
• 90% resistance to Poison (380000 damage per turn)
  • 10% more Poison resistance than level 7
• 25% resistance to Weaken
• BC Drop Rate base reduced to 22.75%
• BC Drop Rate buffs reduced by 66% (roughly divided by 3)
• HC Drop Rate base reduced to 2.5%
• HC Drop Rate buffs reduced by 80% (roughly divided by 5)
• Crit Chance buffs reduced by 60% (divided by 2.5)
• Immune to crit damage buffs, but not base crits
• Immune to elemental damage buffs, but not base elemental advantage
• Spark damage buffs reduced by 50% (divided by 2)
  • 10% more spark resistance than level 7

Elgif regenerates between 50000 and 60000 HP per turn (10000 more than level 7).

Level 10 Elgif has a 35% chance of dropping, up from 20% for level 7 (unknown chance for spawning, as other bosses may appear)

Elgif's AI is mostly the same as before. There's two major differences, and the rest of its differences are in its moveset.

The first major AI change is this: Elgif level 10 will respond to the player activating overdrive or using a UBB by nuking for every turn a unit is in OD state or has used a UBB. After the nuke, the Elgif will buffwipe your squad. This nuke is only active for the first two turns of the fight, but does not interrupt the Elgif's fixed skills in those two turns, nor prevent any RNG AoE from occuring.

• In other words it's safe to UBB from turn 3 onwards, but you can't even activate OD before then or you'll get nuked.
• The nuke does about 30000 base damage. It has all elements, so unless you have elemental immunity it's closer to 45000 damage. Considering you'll be buff wiped after the nuke, it becomes very difficult to survive any random Genocide Tablets if they happen (Elgif will have one action left after scripted stuff happens)
• If you successfully poisoned the Elgif it'll use its last action to give itself regen, so you just need to survive the nuke... and have your buffs removed regardless.

The second is this: On turn 1 and turn 2, the Elgif will activate its damage reflect DoT. This is earlier than the level 7 Elgif.

The rest of the differences for this Elgif are in its moveset:

• Each AoE does a bit more damage than before (25% more due to the higher stats)
• Elgif's damage reflect buff now also gives the Elgif heal-when-attacked, making it bulkier
  • Repeats on a 3 turn timer
• Last Resort (50% threshold that increases hit count/damage massively for normal attacks) now gives the Elgif 10% additional crit chance and 50% mitigation
  • Repeats on a 5 turn timer after 50%
• Poison actually does significantly more damage to level 10 Elgif compared to level 7 - they both regenerate about 200k HP per turn when poisoned, but poison deals 380k to this Elgif (it deals 220k to level 7 Elgif, which is barely any difference)