I am an advanced player in Chapter 19. There are three types of battles:
1. Provence battle - they tend to be trivial - I always win often with few losses.
2. Tourney Battles
2a. the first 8 are easy, on the first go round, I do not need any help
2b. the next 8 (9-16) are medium - I need some help usually 3 combat buildings and Pet food for a stage 10 Fire Phoenix
2c. The next 8 (17-24) are mix of combat and catering. Your best troops will win but other you need to cater. Losses can be large.
2d. Above 24, are usually not worth fighting, catering is the only option unless it is a perfect fit for battle
3. Spire Battles
3a. Tier 1. Every player should complete this level. difficulty is similar to 2a, but with my normal of 3 buildings and fire phoenix, losses are low
3b Tier 2. typically the fights are winnable with an occasional misfire that needs catering. If you must cater the last item - figure on using 25 diamonds. If I am short on troops, I save them for the last contest and cater earlier contests.
3c. Tier 3. mostly wins by fighting, very rarely do I need to cater, but whenever you cater on the last level, figure you will use 25-50 diamonds
That said, fighting has gotten more difficult. To compete, you need a lot of items via crafting and the spire - 3 Pet food a week and 3 combat building a week. Lately, it looks like 4 combat building might be needed. So what does this mean? Crafting is the key to success in battles at my level. Lots of CCs are required as well as time instants for troops. I do 30 provinces a week in the tourney but in chapters 8, I was doing 6-8. Most were via catering. Basically, until you can be effective crafting and getting a high level Fire Phoenix and Brown bear - fighting is really hard. My experience was that I was far better off catering in the earlier levels. My experience early on was that getting enough troops was hard and they always lost until you built the right AWs.
So what am I saying? If you are fighting without help, it is no surprise that you lose. The game is based on advancing, advancing your AWs, advancing your regular buildings, using expiring building effectively, evolving the most important two buildings in the game (Fire Phoenix and Brown Bear). And of course, choosing the right troops in each battle. Elvenar is a slow game and not easy. Being space efficient is not easy. Throwing away old less efficient buildings is not easily done by many. I play to advance as rapidly as possible, that is my sole objective. I play daily so what works for me will not work for others. But this is a good game and I don't actually see the combat system as broken. I see everything in Elvenar as hard and mostly hard to balance all the competing objectives. One of those is balancing catering with fighting. If you are doing just one after about chapter 10, I suspect you are not optimizing correctly.
I will add that I am human and found some troops are far better than others. The right troop type reaches three types rather than just 2 - Priests are the best type for humans by far and can kill a few light melee units. Rangers are more effective against Heavy Melee units than priest and have greater success against heavy range strikers than other light range strikers. Frogs are the best heavy range strikers but still are not very good. Paladins are marginally the best heavy melee units. Cerebrus are the best of the light melee units but expect heavy losses. My experience is that fights requiring mostly Mages and Light Range Unit are easier. Levels requiring mostly Light Melee and Heavy Range units are the most difficult. Most of the time, I use 3 of the best type that is effective against 4 units and 2 that get effective on 3 units - this is especially true if a pair of units is the same type as the unit with advantage of 4 units. Of course, if one unit type is effective against all 5, I take that. The only exception is Mage units. If I have 3 heavy Melee and 2 light melee, with combat building - it is a perfect situation where boosted mages often win with ZERO losses.