Im pretty sure I found the sauce!

• SauceNao SourceURL:https://www.pixiv.net/member_illust.php?mode=medium&illust_id=85607659
• Title:休日のツンデレ巨乳スライム子ちゃん
• PixivId:85607659
• MemberName:脱穀次郎
• MemberId:58552278
• Similarity:94.83

Found On Trace.Moe with a similarity of 0.89: - Title English:Arpeggio of Blue Steel

• Title Romaji:Aoki Hagane no Arpeggio: Ars Nova
• Title Native:Arpeggio of Blue Steel
• anilist Id:18893