This page shows the defense skills that only A-Gears have:
Barrier: 
Level required to learn: 30
Purchase price: 14000 SPI
SP cost: 40 SP
Duration: 15 sec
Cooltime: 60 sec
Invincible to Advanced Weapons. Can only be used in the ground.
Damage Absorption: 
Level required to learn: 15
Purchase price: 10000 SPI
SP cost: 15 SP
Duration: 180 sec
Cooltime: 170 sec
Decreased damage from collisions: +50%
Remedy: 
Level 1
Level required: 45
Purchase price: 15000 SPI
SP cost: 40 SP
Duration: 9 sec
Cooltime: 10 sec
Recovers shield by +800
Level 2
Level required: 52
Purchase price:
SP cost: 45 SP
Duration: 9 sec
Cooltime: 10 sec
Recovers shield by +900
Level 3
Level required: 58
Purchase price: 28000 SPI
SP cost: 50 SP
Duration: 9 sec
Cooltime: 10 sec
Recovers shield by +1050
Level 4
Level required: 64
Purchase price:
SP cost: 55 SP
Duration: 9 sec
Cooltime: 10 sec
Recovers shield by +1200
Level 5
Level required: 70
Purchase price:
SP cost: 60 SP
Duration: 9 sec
Cooltime: 10 sec
Recovers shield by +1350
Level 6
Level required: 76
Purchase price:
SP cost: 60 SP
Duration: 9 sec
Cooltime: 10 sec
Recovers shield by +1500
Level 7
Level required: 82
Purchase price: 386000 SPI
SP cost: 60 SP
Card needed: Final Skill Opening Card
Duration: 9 sec
Cooltime: 10 sec
Recovers shield by +1650
Siege Defense: 
Level 1
Level required: 14
Purchase price: 9000 SPI
SP cost: 3 SP/sec
Defense against Standard Weapons: +9.8%
Defense against Advanced Weapons: +9.8%
Level 2
Level required: 25
Purchase price:
SP cost: 3 SP/sec
Defense against Standard Weapons: +14.9%
Defense against Advanced Weapons: +14.9%
Level 3
Level required: 36
Purchase price:
SP cost: 3 SP/sec
Defense against Standard Weapons: +20%
Defense against Advanced Weapons: +20%
Level 4
Level required: 47
Purchase price: 17000 SPI
Required card: Final Skill Opening Card
SP cost: 3 SP/sec
Defense against Standard Weapons: +24.74%
Defense against Advanced Weapons: +24.74%