After receiving this quest you must travel to the "Arena of Souls" in the Southeast (bottom right) corner of the Lunagrant Woodland map. You can find an NPC at the top of the Arena named Celladius the Blademaster. He will offer you a challenge/quest in which you must defeat him. This is a one-on-one fight, he is tough, and the fight lasts 5 minutes, so bring healing and mana potions.
- Note: This quest can only be undertaken by one person at a time. So if the quest is unavailable, you may want to either wait for the person to finish, or just try again later/another day.
Accept the quest and you will be teleported into an instanced Arena area. Click continue on the prompt that shows up, and stand under the statue that appears. The statue will change into different elements (same as God's Trial statues) that correspond with the different elements your skills use (a Vampire will get Dark statue and Fire statue, a Bard will get Water, Wind, and Light statue, etc.). Skills used that are the same type as the statue type will gain a huge damage boost, so it is highly recommended to use same-type attacks to kill Celladius before time runs out. You have 5 minutes to finish him. If you fail, you will be teleported back to where you started the quest and you can just try again. Be quick when attempting this quest because every second counts and it's very easy to run out of time.
After you defeat Celladius, you can take the second part of the quest which is for Resistance Training. You will definitely require health potions, elixirs, healing skills, or even a health spring to survive this quest. You will again be teleported into an instanced Arena, and be given a prompt. Celladius shows up in the middle, with seven elemental statues around the edge. When you begin, he will call out the attack he is about to use. Run under the statue he calls and wait for him to attack. The statues all correspond with the God's Trial statues (Earth, Water, Fire, Wind, Light, Dark) with a new addition: Physical. The physical statue looks like a man in a cape holding a staff (same color as Earth statue, but with no shiny particle effects like the other statues have). If you made it to the correct statue in time, you will take a pretty good chunk of damage, the statues may or may not shift positions, and he will call out his next attack. If you fail, you will most likely die instantly, which is a real death, and you will appear next to the Arena Trackstone. Your quest will update how many more times you have to endure his attacks, so watch that if you want to keep track.
If your class has no healing spells, you may wish to rely on a mount to get around more quickly, and health potions to heal yourself. If your class does have a healing spell, you may just wish to run around on foot and heal when necessary with your skill. Any speed boosting skills or potions can be helpful if you get into a pinch. When the statues shift, they have a slight delay, so it may be confusing which statue you want to run to. Just keep an eye on what they were, then watch for any changes.
When you outlast all of Celladius' attacks you will be teleported back to him. Accept the final quest he gives. Congratulations! You finished the mastery and resistance training. Now return to your skill tutor in Freedom Harbor, and there will be a new option when you speak to them called Mastery and Resistance Training. It costs gold coins to train these, so choose wisely what you want to improve. The first 5 levels (only!) will be reimbursed to you (15G), but anything after that will come out of your pocket, and it is very expensive. Each mastery level up costs one more gold coin than the last level (level 2 to level 3 costs 3 gold, level 9 to level 10 costs 10 gold, etc.). So in total, the cost to level 20 mastery will be 1D 95G (removing the free 15G from the first 5 levels). Resistance training costs slightly less at 60S, but follows the same formula (another 60S per level up).
Once you hit level 20 in your training, your skill tutor will have a new quest for you requiring a certain item. These can be gathered easily inside advanced training ground (level 40 characters) by trading with your Training Master, and selecting essence exchange. Choose the elemental item that gives you what you need (read the description for each item), right click to activate it, and attack the dummy that corresponds. Once you have gathered 15 of these items (probably over several days, so it may be wise to collect these beforehand), return them to your skill tutor and choose whether you want to advance with mastery or resistance. Then proceed back to the Arena and auto-route to whoever your quest says to talk to.