"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)