Tsukasa never gets to Senku because Senku is always aware of his location with the use of scouts and spies.

That said, Tsukasa's "strategy" to wait is idiotic. He and spear guy are enough to apparently wipe-out huge swathes of humans. Just attack during the night and silently kill the two bridge watchers, or swim to the island, climb in, and assassinate Senku.

For a show that tries to pretend its heroes are "smart," it is dumb as bricks.