Big Spirit Guardians buff: You can now speed across the battlefield and deal damage to entire armies

This used to be a common trick that players attempted, and i've seen it tried/asked multiple times at my own tables (in some variations):

The Divine Lawnmower

Get on a mount and ride past an entire enemy army, dealing damage to all soldiers while they can't react; after all it's your turn.

Naturally this didn't work RAW, because enemies took damage at the start of their turn.
And this makes sense, after all you wouldn't realistically take much damage if the spell only touches you for a millisecond.
If this would work, worldbuilding might fall apart a bit because you could just have scores of divebombing fliers sweeping overhead armies with speed buffs for ludicrous speed to deal thousands of damage with a single 5th level caster.

Well, now it does work.

With the new emanation keyword spells like Spirit Guardians now deal damage as soon as a creature enters the emanation area, or "whenever the emanation enters a creature's space".

Now a party can have a cleric that upcasts Spirit Guardians as high as possible, and then the rest can buff them or their mount with speed (Haste is probably enough), resulting in the spell being able to touch every single enemy creature on the battlefield.
And they don't even have to get within 10ft of anyone unless the terrain demands it, so no Opportunity Attacks (and even then, Haste Action for Disengage, or some other ability/spell).

Personally i find this quite dumb, but RAW this is clearly possible.

Granted, enemies can only take this damage 1/turn, but with sufficient speed, this combo seems quite reliable to pull off.

EDIT: Observation by /u/EntropySpark:

It gets worse, another party member on a steed van grapple the Cleric and repeat the same trick, so every enemy is taking this full damage multiple times per round.