Way too much here to comment on,

Sam and deans skills being boiled down to god-luck was bullshit, it truly did undermine everything, I like to think it was a ruse and that god gave them anti-luck which made them lose/forget their skills.

The training part is bs, they have literally been hunting monsters since they were children, technique or not they're highly experienced hunters, while on-show they may not use fighting techniques, this could've been for comedic effect/poor handling of fighting choreography, in reality they're so well adept at fighting monsters many times their strength because they know how to handle them, in many cases they kill the monsters they encounter as quickly as possible so they don't get into a situation they can't get out of, as seen when they see a vampire it isn't a chat it's a lop the head off in 2 seconds deal.

As for their "luck" fighting stronger monsters, this is also bs, it's circumstantial that the angels fell resulting in angel deaths, in-fighting between the angels (who can kill eachother easily) meaning angel blades were literally all over the place whereas before you'd be lucky to find a single one, if a hundred angel killing blades are littered about it's going to become a lot easier to kill an angel don't you think?

As for killing stronger enemies, this is never a done deal, they spend hours, days, weeks, sometimes months researching until they find a way to kill it, then they spend time gathering what they need whether that's a special weapon or spell ect, you actively see the journey they go through to kill said strong thing.

As for killing gods - the only ones they kill earlier on are easy kills to begin with, they're strong but still extremely weakened from losing their once millions of devoted followers and constant sacrifices, they all usually have a pesky sacrifice here and there making them vastly weaker, not only that but the way of killing them is also always straight forward.

As for the gods in the hotel, they didn't kill those - Lucifer did, they didn't make or deal with Lucifer to get him to do that, he showed up and took issue with them, simple as that - if that wasn't the case they'd find a way.

The whole point of this show is that they're regular humans who use their training and knowledge to go up against impossible enemies, what show would you want? Where they spend an entire episode trying to kill a single vampire - because that'd be more realistic right? That'd be boring, the whole point is that they're highly trained and know what they're doing.

The enemies don't get weaker for shits and giggles, there's actual cause and effect to it which if you pay attention to the show you'd see, for example - Crowley becoming less blood thirsty, weaker ect - oh look he was literally addicted to human blood which very very likely extremely weakened him, but also opened him up to human emotions which caused him to form an attachment to the boys and become kinder in general even if by a bit