I would aim to put two more slots into Dragon's Tail. My choice would be an Endred and a Recharge. And Dragon's Tail is Knockdown, not Knockback, so it actually works very well with Invincibility. DT may not be the best AoE scrappers get, but it is a very good mitigation tool, and is frankly the only AoE you get, although my MA scrapper picked up Exploding Shuruiken to help out.
Crane Kick is the only MA attack with knockback, and since it's single target, it won't really be a noticeable reduction to defense. Just don't chase him around. Kick him, then deal with the others, then go kick him some more.
I'm not a huge fan of Aid Self, preferring Tough/Weave, but I would take out one of the IntRdx, and put in a Recharge.
They ALL float down here. When you're down here with us, you'll float too!
@Starflier
I generally slot Aid Self two heal, two recharge, and two interrupt.
NITES, FTW
50s: BS/Inv Eng/Eng Blaster Grav/Rad Fire/Stone Brute AR/Kin Emp/Arch Mind/Sonic Dark/Dark Stalker Fire/Kin Thugs/Poison EM/Inv Dark/Thermal
In works: WP/SS Tank Demon/Dark Claws/Elec Brute Elec/Psi Dominator
If you could live without Jump Kick, Combat Jumping gives a little defense that stacks with the rest of your defense. It doesn't really need slotting, so you'd free up 5 slots to go into Dragon's Tail and other things.
In general, typed defense set bonuses will help you a lot, it's worth looking into.
If we are to die, let us die like men. -- Patrick Cleburne
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The rule is that they must be loved. --Jayne Fynes-Clinton, Death of an Abandoned Dog
With the recent buffs to both MA and Inv, I pulled my Lvl 50 scrapper out of mothballs and have been playing with him recently. I forgot how much fun it is kicking bad guys in the head. 
I was able to fit both Aid Self and Tough/Weave in my MA/Inv build, and it made a big difference in survivability. As a drawback, I had to sacrifice the APP powers, but I don't really miss them.
I noticed you have your attacks each slotted with two Accuracy enhancements. I think with the To-Hit buff from Invincibility, you can get by with one Acc. Then you can add a Recharge or EndRdx enhancement in your attacks.
If you need more slots, I would take out the second EndRdx out of Unyielding, or Heals out of Health and use them elsewhere.
The use of IO's really help. Even ignoring the set bonuses, getting seven or eight enhancements out of six slots really make my scrapper more playable.
your def/res numbers are a little low, the single best thing you could do to raise them is pick up Tough & Weave, which you could make room for by dropping LBE, SJ and Unstoppable.
It'd put you at around 30% def and 70% res to Smash lethal, more with IO bonuses, which is comparable to an IO only Granite.
Just my two inf,
I made a MA/INV a long time ago, Pre-ED and GDN. Afterwards I was never able to get him quite back to where I wanted. MA damage always seemed low and the knockback on dragon tails hurts invincibility. INV, without parry (Kat or BS) or a heal (DM), seems to die rather easy over a long fight. I'm almost convinced that MA/INV is one of the least effective scrappers, but I love the look and play of both of them so I'm really trying to find a build that works. Currently, he is level 41 and I'm not worrying about set bonuses with him yet so I just used SO.
I picked up Aid Self to get a bit of healing. I prefer super jump to flight so I picked up Jump Kick rather than AS. RElem and REng give more damage resistance than I remember. I don't know if they got buffed in the past few patches or what so I picked them up. I still have some endurance problems over long fights so I picked up Conserve Power.
Hero Plan by Mids' Hero Designer 1.401
http://www.cohplanner.com/
[u]Click this DataLink to open the build![u]
Level 50 Natural Scrapper
Primary Power Set: Martial Arts
Secondary Power Set: Invulnerability
Power Pool: Leaping
Power Pool: Fitness
Power Pool: Medicine
Ancillary Pool: Body Mastery
Hero Profile:
Level 1: Storm Kick -- Dmg(A), Dmg(3), Dmg(3), Acc(13), Acc(25), EndRdx(42)
Level 1: Resist Physical Damage -- ResDam(A), ResDam(5)
Level 2: Temp Invulnerability -- ResDam(A), ResDam(5), ResDam(7), EndRdx(25)
Level 4: Dull Pain -- RechRdx(A), RechRdx(11), Heal(34), Heal(34), Heal(45), RechRdx(46)
Level 6: Jump Kick -- Dmg(A), Dmg(7), Dmg(9), Acc(15), Acc(40), EndRdx(42)
Level 8: Crane Kick -- Dmg(A), Dmg(9), Dmg(11), Acc(15), Acc(40), EndRdx(43)
Level 10: Hurdle -- Jump(A)
Level 12: Focus Chi -- RechRdx(A), RechRdx(13), RechRdx(39)
Level 14: Super Jump -- Jump(A)
Level 16: Unyielding -- ResDam(A), ResDam(17), ResDam(17), EndRdx(34), EndRdx(36)
Level 18: Health -- Heal(A), Heal(19), Heal(19)
Level 20: Stamina -- EndMod(A), EndMod(21), EndMod(21)
Level 22: Crippling Axe Kick -- Dmg(A), Dmg(23), Dmg(23), Acc(36), Acc(37), EndRdx(43)
Level 24: Aid Other -- Heal(A)
Level 26: Dragon's Tail -- Dmg(A), Dmg(27), Dmg(27), Acc(40)
Level 28: Invincibility -- EndRdx(A), DefBuff(29), DefBuff(29), DefBuff(37), ToHit(48)
Level 30: Aid Self -- IntRdx(A), IntRdx(31), IntRdx(31), Heal(31), Heal(42), Heal(46)
Level 32: Eagles Claw -- Dmg(A), Dmg(33), Dmg(33), Acc(33), Acc(37), EndRdx(43)
Level 35: Tough Hide -- DefBuff(A), DefBuff(36), DefBuff(39)
Level 38: Resist Elements -- ResDam(A), ResDam(39)
Level 41: Resist Energies -- ResDam(A), ResDam(45)
Level 44: Unstoppable -- RechRdx(A), RechRdx(45), RechRdx(46)
Level 47: Conserve Power -- RechRdx(A), RechRdx(48), RechRdx(48)
Level 49: Laser Beam Eyes -- Dmg(A), Dmg(50), Acc(50), Acc(50)
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Level 1: Brawl -- RechRdx(A)
Level 1: Sprint -- EndRdx(A)
Level 2: Rest -- RechRdx(A)
Level 1: Critical Hit