The white shield can only block physical attacks, not spiritual attacks like the Life Siphon divine ability. So he felt the impact of the divine ability as soon as he was struck with it.
For mortals, the Life Siphon divine ability would absorb their life energy. But the giant is not a mortal. He is a divine being with divine energy. So, what the Life Siphon divine ability was siphoning from him was divine ability.
The amount of divine energy that Legion-2 was expending on the Life Siphon divine ability was far less than the amount of divine energy that he was draining from the giant, so he was gaining more divine energy than he was losing.
In the overall situation, the amount of divine energy that Legion-2 was siphoning from the giant is about equal to the amount of divine energy that he was spending on all of the divine abilities that he was using.
