1. HMB
2. Guide - Xen Lab Optimal Results

Normally, quitting a battle early will gives your units some experience for missions that cost energy.

In the case of Special Daily Missions like The Fortress and The Threat, this consumes one of your free attempts.

Xen Labs however, will consume the energy cost of 4, but will not give your units experience for quitting early.

Instead, it leaves all your Xen Lab units is the same state they were in prior to attempting the battle.

Since every battle has randomness in it, this means the outcome in one attempt can be very different for another attempt; even if you started out with the same units, from the same condition, and making the same first move.

You can repeat a Xen Lab level as many times as you want (at the cost of 4 energy per attempt) till the level ends in the most favorable condition(s); like having most of your unit skills off cooldown by the end of battle, or taking the least damage possible.

Note: To quit a battle, click/tap the button on the top left which will bring up the menu with the "Exit battle" option.