Specifically, what % of the area they strike and how often.

Method: A setup with a bunch of watchers (I forget how many offhand, but it is in the scripts) and logging how often they swing and how many watchers they hit each time, then turning that into a % of the battlefield.

Frequency seems to always be 320 ticks. The debug log spits a message every 1000 ticks, the 320 comes from the fact that there are seven 1000 tick "buckets" with three swings followed by one with four. This also comes up as the number of ticks the game decrements stun effects by in the debug log.

Takeaway from this is that battle enchantments swing 7500/320 = 23.4375 times per combat round.

Without going into detail, the number of squares per swing were simply obtained from letting the battle run to turn timer and analysing how many watchers were hit.

FV 1%
Rigor 3%
Fire storm 3%
Acid storm 3%
Wrathful skies 0.3%, doubled if storm
Wailing winds 5%
Soul drain 3%
Solar brilliance looks like two separate swings of 2.5% each
Rain of stones 1% per square to have a rock fall on it
Meteor shower every swing, make 5 rolls at 15% and drop that many meteors
Heat from hell 3%
Grip of winter 3%
Astral Tempest 2% damage and 4% interrupt (I assume this is what eff129 is)
The Ark 3%

Meteor shower is a bit special. The number of meteors summoned per swing varies (blue below). This seems to roughly correspond to 5 rolls at 15% each to drop a meteor (orange below).

Data and scripts:

https://drive.google.com/file/d/1tlnBjixG25-80VJd3DzgqyZ-QhViv30W/view?usp=sharing