After Ainz had his small mental breakdown, he excused himself to continue doing an inventory of Nazarick's treasury. They had to know what resources they had available and in what quantities now that getting many of them was impossible.
The entire Nazarick was focused on internal information gathering and would be for weeks to come until Ainz had a clear picture of the Nazarick's resources.
Rei wasn't concerned with that, though. He never considered Nazarick's treasury as something that belonged to him or even as something that could benefit him. He knew Ainz would never squander anything inside because of some nostalgic crap so every time Rei donated to it, he made a peace with the fact he was basically giving things away for free.
For Rei, only things in his personal inventory, warehouse, and... in one other place... mattered. He only counted as 'his' resources things that belonged fully to him.
Not interested in Nazarick's resources, Rei decided to ignore the happenings in the Tomb and continued with his plan for the day, dragging the reluctant Solution into the Amphitheater on the sixth floor for their promised duel.
"I am still not sure I want to do this..." Solution, who stood opposite Rei in the colosseum, said, her discomfort plainly visible in her body language.
Rei gave her an encouraging smile in response. He knew that for her, attacking him even if it was in a spar was tantamount to betraying her purpose in life. Unfortunately, Solution was the only being Rei could spar against without worry. He trusted her and knew she could take most of what he was able to dish out and that meant she would simply have to get through her issues about sparing with him.
"Don't worry, the colosseum is enchanted to withstand a barrage of tenth-tier spells." Rei tried to reassure her.
"That's not what I am worried about." Solution quietly muttered with a frown on her lips, causing Rei to chuckle.
"Ready?" He eventually asked, shifting his weight to his right leg in preparation to instantly move while focusing on his silky white gloves that were actually divine-class custom-made weapons capable of enhancing his spell-casting.
Solution took a deep breath, hardening her resolve as she weighed her silver german mace and kite shield in her hands. She existed to serve her Master and if he desired to fight her, she would oblige no matter how distasteful she found it. Nodding, she spoke, "Ready."
The second Solution uttered that word, she silently cast a Mana Shield around herself and as expected, not a moment later, Rei's Combustion spell impacted the newly created Mana Shield with an ear-shattering explosion.
If the seventh-tier [Combustion] impacted a city gate, it would rip it out of its hinges and send it crashing through the air for at least a hundred meters at a breakneck speed. Yet, with Solution's resistances and fifth-tier [Mana Shield], her feet barely slid back a meter on the ground.
As expected, there was not a scratch on her.
Rei knew that nothing short of eight-tier would be able to injure Solution. He made her durable enough for that. In theory, the seventh-tier should do it too but with her magic defense, it would be a difficult thing to accomplish. His seventh-tier [Combustion] was basically just a warning shot. It was simply supposed to not-so-subtly tell her to take it seriously while also experimenting with the spell's effect in this new world.
In Yggdrasil, [Combustion] was an offensive spell. It would never be capable of pushing somebody back as it did right now with Solution. It simply did the damage. Now, however, it was different. The kinetic force produced by the explosion clearly had aftereffects.
'Well, that is one test done.' Rei mentally nodded to himself, pre-casting teleportation spell.
Solution started her offensive lightly with a seventh-tier [Holy Light Barrage], which produced seven white beams zipping through the air straight at Rei.
Straightening his back and aiming his palm in front of him, Rei used sixth-tier [Air Barrier] to eat up most of the offensive power behind the spell while also using the natural advantage of air against light-based spells. This was a somewhat wonky thing because air didn't officially counter light. It was simply one of these things the gamers noticed during the game that light-based spells had a bad time dealing with the air element.
Normally, Rei would use a fifth-tier spell to minimize his mana expenditure if somebody attacked him with a seventh-tier light-based spell. But this was Solution and he knew how much of a punch she packed behind her spells.
The seven beams impacted the Air Barrier and Rei felt as if a truck slammed straight into his palm. He was momentarily stunned at the sheer might of the seventh-tier spell but also at the strength his new body possessed. The Air Barrier didn't manage to stop the seven beams completely, though. They passed through it, vastly diminished, and either missed Rei's body, impacting the wall behind him or smashed right into Rei who...
'The Law of Energy still applies, huh?' Rei mused, 'The spell was spent to push through my barrier, lowering its offensive power.' He didn't even feel it since the diminished spell could be barely comparable to fourth-tier.
Rei suddenly teleported away as his instincts warned him about a magical fluctuation around him. He appeared a hundred meters away from his previous spot, only to see a [Holy Smite] smash right into the place where he stood not a second ago.
Seeing the effects of Solution's [Holy Smite] on the arena, Rei decided to go a bit harder as he saw the colosseum could take it. Unlike on the grassy plains near the Carne Village, there was no earthquake produced by the spell as the ground of the colosseum was far too sturdy for that. Shockwave and fierce winds were still present but these hardly bothered Rei or Solution.
"[Create High-Tier Earth Elemental]" Rei exclaimed, trying to control how much mana he channeled into the spell.
The ground near Solution shook, almost unbalancing her. Cracks appeared near her when suddenly, the earth shifted into a bulky humanoid fifteen feet tall figure that instantly attacked Solution.
Solution swung her mace at the approaching earth elemental, idly wondering why it was so small... and blasted right through its shoulder as the shield impacted it, creating a large dent in its body made of stone. Contrary to Solution's expectations, the Earth Elemental wasn't bothered by its 'wound' and she had to use her kite shield to deflect its remaining hand mid-punch. Obviously, Solution was far more powerful than the Earth Elemental and as such, its remaining hand got obliterated upon impact too.
That didn't matter, however.
Solution's eyes widened as she realized Rei had ample amount of time to prepare an attack while she fooled around with the Earth Elemental. On instinct, she spun around, her shield in front of her body. It was a good call as it saved her from being hit by Rei's fifth-tier [Earth Bullet] spell and losing this round of the spar.
After all, the first one to land a solid hit on the other would be instantly the winner. A rule Rei established because this spar was aimed at trying out the effects of various spells rather than just playing magical ping-pong with shit capable of wiping out a city at each other.
As the Earth Bullet lost its velocity and the spell became harmless, Solution straightened out, causally swinging her mace to the left and smashing the handles form of the Earth Elemental to pieces while frowning at the wrinkles on her maid uniform. Her mind was running a mile a minute, thinking how to catch Rei off-guard. She had over a thousand and five hundred spells in her repertoire courtesy of Rei, now she only had to decide which one to use and that was proving to be harder than she expected with so many available choices.
'This... is actually quite fun.' Solution smiled. 'I am starting to understand why Rei wanted to spar. It's hard to come up with the right counter-spell on the fly.' She realized she somewhat lacked in this aspect and her determination rose.