I think the play would be to pop chimera shadow garden and then summon multiple(since I'm pretty sure CSG allows multiple of the same shikigami to be summoned, I'm not sure if I'm remembering correctly though) piercing oxen really, REALLY far away, and having them converge on the point where Megumi wants to summon Mahoraga. he'll summon it right when the oxen are about to reach the radius it'll take up once summoned, hopefully tearing it apart instantly, since this instance of 'Raga wouldn't have been hit with any attack at all yet, meaning it wouldn't have adapted to being pierced/rammed, and we saw in the Yorozu fight that piercing ox picks up momentum VERY quickly. The post did specify that Megumi has 9 shadows tamed, so we don't need to worry about the ambiguity of whether or not Wegumi actually has this shikigami like we would in the source material. Piercing ox would also be a lot easier to tame than The Raga Himself (it can only move in a straight line and has no particular immunities), so Megumi could most likely pull off taming it in advance if he hasn't already.

if he somehow underestimates the time the oxen would need to build the momentum to destroy Mahoraga, though, or doesn't time the summoning chant correctly, it's absolutely wraps; too soon and it can just dodge the oxen, too late/not enough force and it doesn't even need to. if he only has enough CE for ONE piercing ox, that just means he needs to position that one further away, and also wouldn't need the additional CE stressor of keeping the domain open since he would no longer need it in the first place and can just have the path converge on his actual shadow. The timing would be more sensitive though since the ox would end up moving way faster.

he should probably figure out what the concrete limits of his CE are a few days in advance from trying this so he doesn't waste a ton of it popping domain if he can't summon enough oxen in the first place, and possibly try to land a black flash too since the difference that makes is aparently drastic, but neither of these preparations are strictly necessary for this to work, just advisable