Well, I knew this might happen.
I was also actually considering changing the defence modifiers to (MAG+SPI or END+STR). The damage modifier does need to be changed, I agree. I'll change the first modifier to x10.
You explicitly cannot die from using Physical attacks. You simply won't be able to use more health than you have to attack. You will be given healing items and you do have a medic or three anyway. You also have weapons with which to beat enemies with- useful for when your attacks are resisted.
The wording is correct; it's a +20 to the damage you would've dealt. I've done some calculations (using Void's suggestion):
Given an enemy of about 30 SPI (300 HP), with no multipliers, Agi from a Magic user (say, 30 MAG) should deal, assuming an average of 20 MAG, about 45 damage. Fire Boost boosts that to 56 damage, and fire weakness to 62. That should be alright for combat speed- assuming not every enemy is weak to fire-, although it might need some tweaking.
As for defences, you will also get some armor that deducts a flat amount from the damage taken overall. Durability shouldn't be too much of a concern just yet. Likewise, the skill trees should have skills which boost stats anyway.
Healing will probably receive an overhaul anyway. Might reduce the percentage it heals and add a different calculation to it, so hold tight. Likewise, all Magic spells might receive a change in cost from a percentage to a flat cost.
Everyone awaiting move choices, you'll be given them soon as I get out of bed.
EDIT: Healing calculation is now Base Healing + (Mag x 2) x d50%. Dia now only heals 5% to compensate.
All non-Physical skills now cost a fixed amount of SP. This is to benefit Spirit stats in Magic users.
Anything that cost 7% SP now costs 9 SP.
Anything that cost 12% SP now costs 12 SP.
Please make the appropriate edits. Again, I apologise for the inconvenience, this is the first time I'm testing this system after adapting it.