Haruhi Suzumiya Vs Lord Of Nightmares
- No holding back
- All serious and bloodlusted
- The power to erase their opponent is banned
- Different universe
- Win by death
Lord of Nightmares, both are sometimes called omnipotent, but Lord of Nightmares has far more feats under her belt to attest to her power, like creating (at least) 4 different multiverses.
(I put don at least because I'm not too sure about the canonicity of knight of the aqua lord, if it's considered canon, then they are countless multiverses, not only for as the novels implies).