Spray the beetles with soapy water and jet-spray them off with the garden hose. That's the most environmentally friendly way to do it. Plant some garlic or onions with your lilys. The aroma should repel them away.

I find picking them off and squishing them regularly is the only way to get rid and that is only temporary. They pupate in the soil and emerge in the spring. The adults fly in from other areas too. The beetles don't usually kill the bulb so if they have failed that could be due to them rotting or perhaps vine weevil. I don't find garlic deters them at all sadly.
I have noticed a small parasitic wasp flying around them so we can hope there is a natural parasitic wasp to predate on them.

Check under the leaves for small 1mm reddish oval eggs when you have seen adult beetles and then for grubs that are covered in their own faeces [sludgy brown grubs].

If you wash them off without drowning them they will still be on the soil and probably belly up and very hard to see as the underside is soil coloured.