Elgin didn't last long, which isn't that surprising since there are multiple enemies to be dealt with this time around. The MVPs of this fight were Nasiens and Donny, so I suppose Anne, Percy and perhaps Sin will get to shine in the upcoming fights. Anne chided Elgin for hitting a girl, so perhaps she will have to face the female knight in a duel to spare the boys the shame of having to beat up a woman. The effects of the Cernunnos' horn seemed like a potential ace in the hole, so I'm a bit surprised that it was utilized this early on in the battle. The usual pattern in shounen is that the same trick isn't going to work again, which means the gang could be in trouble now.
For some reason I had the impression that Nasiens' magic was only about neutralizing poison, infusing his weapons with poison or creating clouds of poison gas to attack enemies with, which seemed a bit limited. The ability to also buff himself to a significant degree opens up some interesting possibilities. If he can use different buffs to boost his strength, defense, speed, reaction time and so on, then he's got a very organic way of growing stronger over time as his arsenal of alchemical ingredients expands. Perhaps the right combination of magical effects could eventually even allow a normal human like him to match demons and goddesses in power. And all of this is not even taking into account his ability to effectively copy the effects of certain other magic abilities. I guess in that sense he's kind of like a mage who can learn all kinds of different spells for different situations. There might be some limitations to his magic in terms of how many effects he can combine or how long the effects last, but I don't know if that's really necessary seeing how Nakaba was fine with keeping someone as broken as Merlin as part of the main cast.
Also, does the Fountain of Youth still exist in some form? If Nasiens could get his hands on that, he could potentially produce a limitless amount of the elixir of life or at the very least become one of the best healers in the series.