Some things to know:
The skeleton's name is Momonga, but he adopted the name of his guild after coming to this new world, [Ainz Ooal Gown]. His comrades had all since quit the game and left behind their equipment for Momonga to use. Momonga built his character for fun, and specialized fully into roleplaying as an Overlord of Death.
Shalltear Bloodfallen is an NPC created by one of the guild members, Peroronchino. Perorochino is known for his obsession with min-maxing, and as such, Shalltear is created to be an incredibly powerful anti-player character, and was the first line of defense against player invasions in the guild hall, Nazarick. She is created to be unquestioningly loyal to all members of Ainz Ooal Gown, but she was mind-controlled by an item of the absolute highest tier, one that ignored her complete immunity to mind control. Before the mind control took effect, she struck down the person who used the item on her, so she wasn't given any orders and was put into a passive state.
Momonga uses two types of cash shop items in this fight. The first are the wooden sticks that allow him to instantly change his equipment. They do not grant him any special equipment, but can be thought of as a hotswap enabler. The second is the hourglass that instantly completes the casting of a super-tier magic, all of which have very long cast times.
Besides her equipment, the only special item Shalltear has is an automatic resurrection item.
You may call Momonga's battle plan pay to win at first glance, but keep in mind that this is no longer a game for him. His game turned into a reality, and he lost all metagaming functions (things like contacting a GM, menus appearing, etc). Death is the outcome of losing, and so not doing everything in his power to win would be suicidal. Shalltear, however, was revived at the cost of gold back in Nazarick.
Skythewood did the translation for the light novel, and I absolutely recommend you read it. It's a twist on the "player's game turned into reality" gimmick, where the player is already at max level with the best possible equipment and an underground stronghold, and the world's inhabitants are unfathomably weak compared to him, or any of his NPC subordinates.