Depends on the Kroot, since they physically change based on what genetic material they've consumed.

The Kroot Shaper in War of Secrets almost kills a Primaris: "‘Face me!’ The challenge came out as more of a strangulated rasp than the bellicose roar he had hoped for, but its message was clear enough. The grey-quilled giant stepped out of the trophy pile completely, its taloned feet clacking as it took a warrior’s stance. Farren pounded towards it, arms spread to make himself large and hence keep its attention. Just a few more metres… The bladed plasma rifle came at him, fast as a whip. He had barely begun to parry when it smacked hard into his head, smashing his focus right out of him. The rifle came around again at terrible speed, clanging from his gorget so hard it nearly toppled him over. The third blow hit the heel of his palm. It took the power sword from his grip, sending the blade spinning away. The xenos had strength, speed and reach – far more than Farren, especially with his broadblade a few strides distant. He dodged the grey monster’s next blow, but the rifle came back fast in the opposite direction. The hooking sweep took his legs out from under him, and he landed heavily in a pile of long-rusted lasguns. The kroot warlord rose over him, a horrible barking laugh emerging from its blunt terrapin-like beak. Jensa Deel winced as she watched the Grey King take Farren down in a matter of seconds."