To Shield Accelerator:
Additional 50% in defense stats does not mean half damage.
Simple example, an attack of 100 vs. a defense 20, 100 - 20 would be 80 damage.
50% additional defense would mean +10 defense, 100 - 30 would be 70.
Which is MUCH different from half the damage of 100, meaning 100 / 2 would be 50.
Of course, formulas are much more complex, but this is simple math and comprehension.
The skill itself isn't worth it. Rena's defense is already lackluster, another 50% of lackluster is nil. Better off getting Magic or Power Accelerator, or putting it in random utility area.
Also, Assault Kick isn't that bad to get. As opposed to Spinning Kick, gained from Combat Ranger, you can combo off Assault Kick as a 100 MP skill. Also, for the minimalists, if you get Assault Kick over Rail Stinger, you don't need to spend points into a buff to unlock the next tier.
Aero Tornado is decent, but it's not a must-get, especially for PvP. The skill will only prove itself useful in PvP in a mass-brawl game, where there's clumps of people and not all of them are going to chase you down. Keep in mind, PvP should mostly focus on low-MP combos more than high-MP combos, because you very rarely should get the MP for 300 MP skills (unless someone is just that bad versus you).
As for PvE, you will most likely get more damage out of Siege Mode X-spam than using all 300 MP. Again, this skill only proves itself great for quick damage and when your boss is piled high with minions.
The acquirement of Aero Tornado ultimately boils down to how one wishes to use their last 5 skill points to unlock the next tier, as people often choose between Phoenix Strike or Aero Tornado. There's absolutely no use getting both either.