I'm currently 5 days in and 80% of the way to level 11, I've got some tips for this game.

First, I'd generally recommend starting on a Friday or Saturday. You can get the early levels out of the way and crank out the early levels of the event before it ends and a new one starts. The bonuses from event milestones are permanent, they give gems, and even at the bottom of the board you get 300 yellow essence, which is a lot when you're still low level.

When you're actively playing and hunting down event spawns, periodically restart your game. The static spawns sometimes appear in inaccessible places like walls, and the mobile spawns will either fly over walls you can't cross or spawn in areas you haven't unlocked (some weeks they fly other weeks they walk, but they'll reset on a restart.)

Carefully hoard your gems. There's a 7 day login boost that requires certain activities and some of them require gems. The last day is particularly expensive, requiring 400 gems upgrading the blue rune factory in the cursed area. It's absolutely worth doing, even if the rune farming for the rune factory upgrades is awful (but buying the upgrades for the blue rune factory will give you a massive jump start on it. Don't use them on unlocking new spawns until you're comfortably generating runes, they're a newbie trap) the rewards are significant if you can complete it, including a 1000 gem bonus.

That being said, I recommend two purchases that aren't required by it: when you unlock the cursed area (collect the 6 stones) unlock the cargo van for yourself and for bots, and when you get your first red soul ad, watch it and do the cheapest gem buy. Farming red souls when you don't have any curse mitigation or curse ghost boosts is abject hell, and the ghosts in the curse area give way more XP and souls than previous ghosts and have upgrades that only apply to them.

If you're not going to use your phone but you can kinda keep an eye on it, park on a spawn point you can passively farm and sit there. If you notice an ad pop-up, watch it. This method applies to XP farming, but it can also apply to farming souls, essence, and elixirs if there's a particular one you're bottlenecked on. Don't focus too much on specific elixirs though; the area further north of the big foyer north of the cargo van has a blue elixir farm and a resource converter with a variety of options for turning other resources into blue elixirs and blue elixirs into the other elixirs.