SMITE

"Use your Rewind ability while you're below 15% health and return back to full health."

Rewind is Chronos's Ult, which allows him to go back in time restoring his health to that point in time (plus some cooldowns stuff, but that doesn't matter for this)

Here's my easy way to do it:
You have to have full health before walking into the tower. Also, his Ult has a delay 'til it activates so use it right before the final shot hits you (The amount of shots you can take may vary)

As Chronos, use your Rewind ability while you're below 15% health and return back to full health.

Rewind is Chronos' ultimate, and it resets him to wherever he was eight seconds prior, as well as resetting his cooldowns. The problem is that there's roughly a one second channel time before rewind activates. Because of this, you'll need to read the situation. If you get hit by a long range attack, like Vulcan or Kukulkan's ult, you can probably stand as far away as possible, get hit, and activate. If you end up wading into a team fight or get picked on by hunters or assassins early on, activate it early since you're taking damage while it activates.

Since you need to be at full health, consider taking healing potions or Meditation, as even a minion taking a potshot can ruin your chance.