Command didn't want that to happen. They didn't want a massive confrontation so soon. They wanted to destroy the power of the elves little by little and stall for more time.
They made their wishes known in the messages they sent to every soldier. At that time, Kelvin was neck deep in fighting.
He wasn't just massacring bugs anymore. He actually had to fight for his kills now.
The enemies he fought with first were the trapper spiders. They attacked him with their webs from a safe distance.
While Life Core worked on them, its effect was finished because they didn't have any wounds that he could take advantage of. The ability was far from lethal to them, so he could only passively dodge their webs or cut them.
The webs were actually very powerful means of attack. They were like bullets with enough power to penetrate his force field easily. It was as if his force field wasn't there.