If, like me, you don't usually use this power during fight, there is an ideal place to boost it.
At the beginning of the act 3 (just after you unlock the Time Rush power), you'll be facing a turret. You just have to activate the Time Shield by pressing
, when it disappears, go behind a cover and wait for the power to charge. Rinse and repeat.
It's very fast: you only have to do that 5 or 6 times.