About Phantasy Star Portable 2 - equipping units?

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I'm LV 79,have A class armor,and use those extension abilities
HP Restore Rate - plus 10% of my max hp every 10 seconds
Photon Barrier - Damage amounting to less than 3% of your total HP is completely nullified.
Damage Resist - Cuts all damage received by 10%.
I play against Earth type monsters and my armor reduces 51% of the damage.

And with all that I still have to be careful when I play against LV55 monsters...what's wrong.

I can still clear solo some missions with LV 105 monsters but it takes a lot of time and careful attacking.
I don't have problems with monsters maybe 40 levels below mine(I have problems with some lv 40 monsters attacks...I can beat them easily but they still hit hard)

I play with A class 9 stars double sabre and sabre which are lightning elemental(string against earth elemental enemys)

Also when I use MySynth,she is nurse type and she gives me 20% fire and earth damage resist...so why?