[How about you summon your Undead Army and surround the boss? All of them will die pretty quickly, but it's fine since you will need a new Undead Army of tier-2 monsters anyway].
'Alright. But this boss is now looking at me, so why is he not attacking me?'
[The fight will start only after you attack].
'I see,' Arkhen nodded as he waved his hand and summoned the Undead Army to fill up everyplace around the boss.
The guardian boss took this as an Arkhen as his eyes glowed intensely for a second, and he waved his hand katana horizontally twice in quick succession as a monotone voice left his mouth.
"Sinking Blade, Falling Star!"
Arkhen's face changed as he felt threatened and quickly flew up while creating force fields around him to defend himself.
*Swoosh-Swash!*
Two slashes! Just two fucking slashes and every single undead monster died! Those thousands of Undead monsters turned into motes as they were killed in one second!
'What the fuck was that?!! Also, he can speak?'