@GreatestSin
Except that the explosion technique is nowhere close to Othinus taking Touma seriously, you should be able to see that's nothing compared to the things she had already done to him in the previous worlds or the things the narrration suggest she could do to him. She never took Touma even slightly seriously until she grew tired of him not giving up and started to feel worn down mentally, finally resolving herself to killing him and continue with her plan, deciding to stop respawning him. Touma uses the experience he had gained through the thousands of deaths to evade the next attacks. This starts to anger Othinus, so between Touma dodging her attacks, the mental damage she's receiving plus his taunts, she turns to Gungnir, the symbol of her power, assuming that would be enough to kill Touma, the "lowly human" who's bothering her. Othinus realizes this was a big mistake just as she throws the lance when she sees Touma's smile; she has underestimated him again. Gungnir is destroyed by IB, so Othinus, now really pissed off, decides to use her crossbow, and with her trick with the last arrow kills Touma.
The bolded part is when Othinus stops playing games and underestimating Touma, decides to fix her mistake and kills him. All the previous parts weren't intended to be a physical competition or a real serious battle, it was a challenge of wills between them to see who would give up first in achieving their goals, Touma or Othinus. This was only possible because of Othinus' doubts about her plan. Just because she was easily killing him before that doesn't mean she was fighting anywhere close to seriously (if she was doing that, she would have never respawned him even once in the first place). It just speaks of the power difference between them when she can kill him without using any of her powers like she does a few times when she kills him using Gungnir as a bat or breaks his neck with one hand. She was treating this like we would treat stepping on a bug, just because we are killing the bug it doesn't mean we are treating it as a serious conflict.
