Immortus had not taken many precautions before journeying to the M'krann. Although he'd of course known about it, and Apocalypse's attempt, for a long time and had anticipated this battle, he'd never imagined that so many beings would come to the M'krann, or that they'd be so varied and powerful. Immortus had battled on like the others, defeating the Sphinx by trapping time around the Ka Stone and then sending the Egyptian spinning through the Vortex of Infinity, and then banishing the Magus from the plain, avoiding a soul-sucking blast from he-who-had-been-Adam-Warlock's soul gem and then trapping him in a branched timeline. But Immortus had been too preoccupied with stopping them and avoiding the many death spells and high tech weapons aimed in his direction--given his unique position many of those on the plain thought it wise to take advantage of the situation by trying to kill him--to stop Apocalypse, and now he was forced to take care of everyone on the plain in a hurry, before Apocalypse returned from the Shining City, at which point Immortus would deal with Apocalypse himself. Immortus could not risk the expenditure of energy and resources it would take to personally kill all those on the plain, so he fell back on his time-tested and honored tactic of using millennia of human troops to do his fighting for him. While most were no match individually for the beings on the plain, Immortus thought they could, by sheer weight of numbers, overwhelm the others. So he brought through troops by the hundreds of thousands, until the plain seemed to teem and crawl with them.
The High Evolutionary's experience and aims and methods were much the same, although his chosen weapons were different. After the failure of the New Dinosaurs to kill the heroes of Earth-309, he'd sent a signal to New Wundagore and began bringing through cyber-velociraptors and enhanced Procoptodon by the tens and hundreds of thousands.