Don't use (Key of(PickedUnit)) or anything like that. Its not efficient. Use:

• Custom script: set udg_YOURINTEGER = GetHandleId(udg_UNITVARIABLE)

Don't use 'Picked Unit' or 'Triggering Unit' more then once. Set it as a variable.

The OB_Targets and stuff is unneeded. Use:

• Custom script: set bj_wantDestroyGroup = True
• Unit Group - Pick every unit in YOUR RANGE range matching YOUR CONDITIONS and do (Actions)

The above script simplifies the steps, and does not require a unit group variable.

• Startup
  • Events
    • Map initialization
  • Conditions
  • Actions
    • -------- ======================================================================= --------
    • -------- DON'T TOUCH THIS --------
    • -------- ======================================================================= --------
    • Hashtable - Create a hashtable
    • Set St_Hashtable = (Last created hashtable)
    • -------- ======================================================================= --------
    • -------- Configure this --------
    • -------- Formula: BaseValue + (BonusValue x Level of ability) = ActualValue --------
    • -------- St_Duration, St_LightningType, St_FloatingText and St_HitsTrees are the only variables that is not affected by the formula --------
    • -------- ======================================================================= --------
    • -------- Damage --------
    • -------- Remember: When dmg is AOE it will deal Actual_DmgAmount PER UNIT, so 3 units is (if Actual_DmgAmount is 150) 450 DMG TO EACH TARGET --------
    • -------- So lower the dmg when you have St_NrUnitsHit = more than 1 --------
    • Set St_CritAmount = 200.00
    • Set St_CritAmountSmall = 150.00
    • Set St_CritBonus = 50.00
    • Set St_DmgAmount = 100.00
    • Set St_DmgAmountBonus = 50.00
    • -------- Area of Effect --------
    • -------- AOE of spell when cast and dmg AOE of the Explode --------
    • Set St_Range = 200.00
    • Set St_RangeBonus = 50.00
    • -------- Sets the number of targets the spell hits --------
    • Set St_NrHitUnits = 2
    • Set St_NrHitUnitsBonus = 1
    • -------- Crit Chance in % --------
    • Set St_SmallCritChance = 20
    • Set St_SmallCritChanceBonus = 5
    • Set St_CritChance = 10
    • Set St_CritChanceBonus = 5
    • -------- St_Duration = 0.00 gives the duration (Until Dispelled) --------
    • Set St_Duration = 0.00
    • -------- Lightning Type --------
    • Set St_LightningType = Chain Lightning - Secondary
    • -------- Booleans --------
    • Set St_FloatingText = True
    • Set St_HitsDestructs = True
    • -------- Mana cost, Cooldown, Cast Range and Description is changed in Object Editor --------
    • -------- Trust me, i wish too it could be done in a trigger. :) --------
    • -------- ======================================================================= --------
• [/HIDDEN]

You could store the level of ability being cast into a variable since you're calling it three times. Not a big deal though.
Save the handle into a variable as mentioned.
I also agree that you should set all the configurables into variables in the init trigger, like base damage, bonus damage per ability level, AoE etc.
OB_Clean shouldn't be on all the time.
The learn tooltip image is missing.
The follow though time is too short for the unit to complete the animation, set it to about 1 second or something like that.
The dummy doesn't need invulnerability ability, is has locust.
I'd set the dummy to can't raise, does not decay. And not remove it with Unit - remove unit from game.
This should be done only when you reduce obcounter, it can't be zero during other loops:

• If (All Conditions are True) then do (Then Actions) else do (Else Actions)
  • If - Conditions
    • OB_Counter Less than or equal to 0
  • Then - Actions
    • Trigger - Turn off (This trigger)
  • Else - Actions

You can replace (Owner of(unit)) with triggering player in the cast trigger. It's slightly faster.
I recommend using bj_wantDestroyGroup for groups that you want to destroy immediately after use. Slightly better performance.
Last created hastable:

• (Load 10 of (Key (Picked unit)) from OB_Hash) Less than (Load 3 of (Key (Load 1 of (Key (Picked unit)) in OB_Hash)) from (Last created hashtable))