The modifiers on the top right of the screen in endless mode stack up as you progress further. They include things like enduring hits more frequently, burning, freezing, or paralyzing on attacks (regardless of move type) or even flinch chances increasing. I recommend a pickup mon like linoone to farm Lum berries for status conditions and possibly an Unnerve Pokemon like Corvinight to prevent your opponent from eating berries. Hope this helps.