Force Field protects the player from the next damaging attack, but only one attack. Levelling up Force Field does not change its effects.
By the time the player acquires Force Field, they also have Barrier and Reflect, which completely outclass Force Field by making the player immune to attacks for a period of time, rather than just one attack. Given how very few enemies at this point of the game use alchemy at this point, even simply Barrier is enough to make Force Field obsolete.
Acid Rain - Atlas - Barrier - Call Up - Corrosion - Crush - Cure - Defend - Double Drain - Drain - Energize - Escape - Explosion - Fireball - Fire Power - Flash - Hard Ball - Heal - Lance - Laser - Levitate - Lightning Storm - Miracle Cure - Nitro - One Up - Reflect - Regrowth - Revealer - Revive - Slow Burn - Speed - Sting - Stop - Super Heal