Basically what I did here was a simple explosives exploit: every keg/bomb blowing up deals damage equal to 10% of max HP to all targets caught by it so you just need to blow up 10 kegs near that spider mech to take it down to 0. The only reason it survived last explosion was because enemies can’t die until you engage them directly (aka when you hear battle music). Since it’s at 0 HP at the start of the fight, it will immediately die by taking any damage.