Play at least 30 games a day so that the cloning chamber resets every day. Always complete bounties for both factions, use fuel cans if the bounties aren’t units in your deck.
Playing that amount will take about 2 full seasons to level common units to 10- low 11. With skill and viable decks that contain only one epic unit you can get to Tiberium towards the last week of the season.
Paying for anything to get crates is a waste. Save diamonds you earn or bought for x5 cloning chambers or the +1000 coins for next 30 battles.
Do not spend real world money to take any unit above 11 unless you intend to spend hundreds of dollars on this game. With an average unit level of 11 you can play the whole month in Masters league. Playing with level 11 units isn’t that bad against level 13 but it’s awful in Tiberium where people role around with level 15 Rhinos like they are a tank.
The only real pain is deciding what rare units work for you and your deck. You can’t really go wrong leveling up any common unit as they all have a use and get used the most. Most epic units can be a level or two lower than the rest of your deck if you are not playing tech.
Basically if your deck is about maxed other than epics in the league you’re in it’s completely viable in the next league. You know your deck is effective in Masters if you can maintain around a 55% win rate on over 100 battles. Otherwise you might want to rethink your strategy.
Playing against higher level units never stops, its part of the game. Most of the time it doesn’t make that big of a difference. For example if you have a Talon to counter flames or shocks it also counters level crazy rifles.
Nod IMO is way easier to advance with, but you have to play both factions to maximize the free rewards.