I know this is extremely off-topic but I want to get a point across, ressistance reduce and elemental damage exists for a reason just like Attack Damage, it's another way to defeat your opponent and not just something that is suppoused to be negated.
If people reducing ressistance to gain an advantage in PvP wasn't meant to be or not acceptable, then it wouldn't be in the game.
Mage class is specifically made to utilize these ressistance reduce attributes to deal damage that may not be possible for us with pure attack power alone as we cant crit, so using ressistance reduce is what gives us an edge on our opponents if they're lacking ressistance.
Now, any user can obtain decent enough ressistance to counter this as there's always a counter measure for almost everything, but as I said before, Elemental Damage wasn't made to just be ignored thus making it an option to defeat the opponent which is why getting the best ressistance is a challenge.
When it comes to how high you get your ressistances, it's really 100% up to you to make that happen, from the start of the game you chose Archer Class which has its set strenghts and weaknesses such as every other class, Archer has the best damage but worst defence and one of the liabilities to Archers weak defence is suffering more to Elemental damage than the other classes as your armour pieces pourpousely have less ressistances than everyone else in exchange for your damage, or another example is Swordsmans having the highest HP amount which then counters Mages low HP amount as they outlast a mage.
Even at most, in order to have a chance at reducing ressistance with the multiple options of gaining ress, a user must have reduce all, reduce fire/water/light/dark and possibly a Shadow stone to keep up with the current meta of things like Fairy Affinity, perfections, SP base ress, act 6 ress etc.
Now from what I said earlier about having enough ressistance to counter elemental damagers, the ressistance needed may not be the most common, but it is still in existance, and it's up to you to work for it as they will always be available options for those who strive to counter Elemental Damage, just as I have to work to make my character as defensive as possible to archers pure Attack damage.
In a 1v1 PvP Elemental Damage becomes risky to build your character on as an opponent with these ressistances will negate you, this is also a counter in the form of ressistance.
But in the same way you have to think of all situations where there can be counters and there cant be, Elemental Damage may be useless vs a foe with full Act 6 ressistances, but in different situations such as Arena or Rainbow Battle your opponents will be forced to use multi-ress in order to have an even balance against multiple opponents, this is where you would think of your options to maniplate that fact and use it to your advantage by going in with a ress reduce set to gain that advantage.
But when it comes to counters there is always a way to counter you opponent, it can even come down to your shell stat having s% increase ress + 10% fire/water ress on the shell, it does seem extreme to search out these specific stats but they exist and are all possibilities and even using ressistance increase equipment and costumes which exist for all elements such as Viking Helmet 10% water, Football Cap 10% Fire, Desert Turban 10% light, I'm not sure if there's one for Shadow ress but there probably will be in the future.
So instead of looking at how much is possible by the enemy, you should be looking for a way to counter what is possible, even if I have to buy multiple armours and weapons for multiple situations, I'll do it so that I'll have a chance to overcome whatever my opponent may be using.
One of my counter robes as an example :