Your PCs might not expect Stalkers approach:Have you actually simulated a combat like that? Think of it this way...
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).
But, if the Koly concentrates its attacks, it downs the first PC on round 2 and damaging a second for 72HP. The party does ~160 HP damage (depending on init) between round 1 and 2. Round 3 a second party member is down (and a third injured for 48HP), and the party now only does ~40 more damage (total 200). Round 4 the party again does 40HP, the Kolys downs a 3rd PC and injured the last one for 24. Round 5 starts with the Koly with 240HP of damage and the remaining PC at 76; it doesn't matter who has init, the PC goes Down and the Koly wins.
Yes there are a thousand possible variations on this based on tactics, features, spells, etc. But this model scales. This example is simply to show you that concentrating damage on a single target is the most effective way, in D&D, to win a battle when other aspects are equal between sides. (Healing can affect this deepening if the healing is for one target or a group. Control spells and feats can influence this as well. etc). This is because in D&D, in almost every case, a PC/NPC does the same amount of damage if they are fully healthy or have only 1 HP remaining. Because of this, the way to live longer is to reduce the total damage your opponent(s) do, and the way to do this is to concentrate damage.
So how you open with this monster, you hit a weapon guy, auto disarm him, and pick up the weapon with your free use an object interaction.
So now unless they have a second magic weapon your damage resistance kicks in. Rinse and repeat on later rounds for any other magic weapon carriers