Just assume all PCs deal the same damage (20 DPR) and have the same hit points (100). If the Kolyarut spreads it damage around evenly like you suggest, then it takes 5 rounds to kill all the PCs. In that time, the PCs do 80 DPR, killing it on round 4. Party wins (depending upon effectivity of EoB and other control effects).
Sure lets look at that with a little more specificity. I think you missed the incapactity of the spread out only edict of blades.
Edict of blades means auto incapacitated for a whole round. So every time it uses that recharge action it damages everybody within its movement range and buys itself another round of attacks where it does not get attacked and take their DPR. It is recharge 5-6 so every third round or so.
INCAPACITATED
• An incapacitated creature can't take actions or reactions.
The Kolyarut has 297 hp.
Say the party gets init.
Round 1 the party goes and inflicts their 80 hp on round 1 so the Kolyarut is down to 217. The Kolyarut then goes and either focus fires on one for 96 (so 4 hp and 100x3) or edict and hits everybody for 24 (down to 76) and incapacitating them.
Round 2 for focus fire the party hits for 80 (kolyarut = 137) and the Kolyarut goes killing one and wounding another for 76 (1 dead, 76, 100, 100). Or the incapacitated party can't take actions. Kolyarut hits everybody for 24 more hp (52 left) or focuses fire and kills one and the other three are down to 76. The Kolyarut is ahead of the always pure focus fire option here.
Round 3 Focus fire the party inflicts 60 hp and K=157 while the Kolyarut kills a second the party left with two at 100. For other options the party goes and inflicts 80 more hp (K=137) or 60 (K=157), the Kolyarut inflicts 96 hp either on everybody (28 hp left for four) or focuses and kills another (2 dead, 1 at 52, 1 at 76).
Round 4 The focus fire party inflicts 40, K= 117, and the Kolyarut attacks number three so the party has 6, 100 hp left. The edicts multiattack party hits for 80 K= 57 or if edicts and focus fire for 20 for K=137. Kolyarut goes maybe edict of blades again on third round of 1 in 3 chances each round. If so the Kolyarut inflicts 24 on each and incapacitates them then goes again killing everybody. If unlucky and no edict at this point the multiattack Kolyarut either attacks everybody leaving them at 4 hp each or focus firing killing a third and leaving one at 28.
Round 5 Focus fire party inflicts 40, K=77, K kills 1 and last party member has 76 hp. Party is either incapacitated and all dies, or four attack and kill K, or last party member hits for 20 K=117 and party dies.
Round 6 focus fire party inflicts 20, k=57, k kills party. Non-focus fire Kolyarut is either victorious or dead with hp at either dead, 57, or 117.
Conclusions: using edicts of blades to minorly damage and incapacitate whole party instead of focus fire is a superior option for when it is an option. Focusing fire instead of spreading out four attacks against four opponents is superior when those are the two options.