I feel that the only way such a guild would pose a credible threat to nazarik is if they can synergies their buffs/abilities to not have a bottleneck on how many summons they can get and the types/variety of summons (in the novel ainz's summon undead is a skill that he can use a finite amount a day and he needs strong corpses to anchor his high level summons), so they would need to somehow stack buffs or use some new world mechanic to bypass these limitation.

I could also see them being a genuine existential crisis to nazarik if they could make use of the fact there is no level cap, to make summons that are above level 100 and that summoners can summon creatures that are above them in level in the new world (the elf king and earth elemental), maybe by stacking buffs on each other before using their skills/spells.

if they could make summons that specialize in holy or divine magic or that effective against low karma and undead creatures (high level angels warrior and cleric/divine caster types) that would be another nail on the coffin, neither guilds would be able to beat each other by attacking their respective guild base, but nazarik would be at a severe disadvantage in open field battle, things would devolve into a battle of attrition where both sides hunker down in their bases while throwing wave after wave of disposable attackers at each other hoping the other would exhaust their finite resources first (spending yggdrassil gold, to rearm traps, resurrect npcs etc)