First off, have you tried the Krakatau decompiler? Krakatau is designed specifically for working with obfuscated Java bytecode, although the decompiler doesn't support lambdas and isn't very user friendly.
That being said, renamed identifiers is not something any tool will able to help you with. You'll just have to reverse engineer it and change them to more meaningful names yourself.
If decompilation fails, you can also try the Krakatau disassembler. The advantage of disassembly is that it always works, no matter how heavily the bytecode has been obfuscated. It lets you see what's really in the bytecode without the risk of things getting messed up by decompilation. The downside is that it's generally a lot harder to read.