Attack it. Your goal is to survive 45 seconds to earn 3 stars/maximum event artifact materials. Don’t try to do damage.
Use support heroes (Goddess, Xak, Green Fairy, Merlinus) and their ultimate skills to help you survive.
Once you get a 3 star attack, just do quick attacks for all the rest. It saves you a lot of time and still gets you all the rewards and benefits
Slay boss NOT up
Can you kill the miniboss? If yes, kill it. If no, summon/search for a new miniboss that you or someone else can kill more easily. You only want to spend 1 or 2 attacks in minibosses to be efficient.
[psy] Grateful Dead 