The level gap matters a ton. If you're a low enough level, your SUPERS literally stop OHKOing people, your melee becomes a 3HKO/4HKO and your grenades probably become garbage too. Fusion Rifles are extremely powerful, especially the 77 Wizard, so of course it can still OHKO you with all 7 pellets connecting. Your abilities seem to be impacted the most by the level gap so if you have a high Attack weapon, even if you're several levels below you can still make a dent.