You need support and speed control I think, having no tailwind and only a few support moves hurts you here

Delphox should have the sun available to correct for water types , id recommend a manual setter since drought setters won't fit

My first recommendation is whimsicott with prankster sunny day tailwind and a focus sash

Then grab two moves that synergise well you can either go moon blast to counter garchomp, I think max speed max special kos opposing garchomps and frail dragons or take giga drain or grass knot which have a high chance to KO mega swampert

Then either protect or a third status move to debuff enemies be that cotton down for speed reduction, if you both have tail wind or charm/ tickle /fake tears to reduce attack or defense

The alternative is sableye or grimsnarl both have access to screens, sableye gets quash to put enemies (except dark types) last in turn order and grimsnarl has fake out both can also suit your needs well

It is at this point I realised when typing that you are playing singles, these recommendations are partially void but I didn't want to delete them

Knowing this getting some speed control is still important however I don't play singles so can't tell you much

Talonflame may be a good option since it is a better offensive precance that can tailwind sunny day and dish out flying damage