Any tips for offering under asking price?

I have seen a house I really like but need to offer lower than the asking price. I would need to offer 20k less, ideally 30k but that might be too far? The EA has said there is some wiggle room in terms of offering under the asking price. I think the house is valued at 10k above the asking price anyway! But my question is, are there any tips for convincingly offering 30k under the asking price? I am chain free so that is a plus but don't what else I could say. Grateful for any diplomacy or negotiating tips here..