The Wiki is finished - and has been for a while but I forgot to say anything
You can see more info about the Vitality Surge
here
And the event itself
here
The Vitality surge is based on the squad size for the battle you are currently in, so a later Tournament battle or higher Spire of Eternity battle will give you the best bang for your buck.
How it works - examples from PC version, mobile will vary but the premise is the same
On your battle results screen, under the units remaining, you will see a green button with a heart on it. Clicking/Tapping this brings up your Vitality Surge instants.
Tooltip popup
View attachment 11640
*3 wave Spire battle used for this example
View attachment 11635
If you hover over the unit, a pop-up will display showing how many troops were lost for each individual slot so you can pick the best option.
View attachment 11636
The display shows what instants you have in your inventory, and what you are reviving as well as how many you lost/had in a progress bar
View attachment 11637
Hovering over the Instants will show you how the revival will affect your squad in the progress bar, in this case, either the 100% or the 75% will revive all of the lost units.
View attachment 11638
You can then proceed to the next wave with all of the Golems revived in the first unit slot.
View attachment 11639