But what about the 2 wave fights, isn't these a lot harder than doing a tournament fight ?
Not really, if it´s an easy fight you have more or less zero losses in round one and more or less zero losses in round two. The only difference is that there are fewer easy fights when there are multiple waves, because you have double the chance of a nasty guy showing up.
As an example: I can fight 20 (or 100) waves of 5 stacks of treants with 5 stacks of Blossom mages, because I will have 0 losses in each wave. The problem is it is not likely to happen that 20 times in a row it´s only treants...
Now in the spire you can always get a max of 3 enemy types (and there are 3 kinds of units for each type)
If we stay with our Blossom mages (my favs) for an example, if the enemies are HM and HR I expect basically no losses, the question is which 3rd type is in the mix? If it is mages, no big problem, the fight will be easy, with minor losses to the mages. if it is LM, we have to look into the details: Is it Thieves or ancient Orcs? Again no problem. The only LM unit that can cause trouble to blossom mages are dogs, those can actually cause losses, but we would have to see the details of the fight (how many dogs, in which round are they, where on the map are they placed, etc to see how serious the problem is). If the third type is archers, again we need to look at which kind of LR, Mist Walkers are major trouble, while Bandits and archers only cause minor damage. Mist Walker need special treatment in most cases, like for example a decoy unit. Overwhelming them works if there is only one in all the waves, but it will always result in losses.
Basically that means only dogs and Mist walkers will prevent an easy fight if the other enemies are HM and HR. if those 2 units are not present I can fight 2 or 3 waves with only minor losses. Of course that really means Blossom mages can be used to fight any of the 5 unit types (as long as it doesn´t include MistWalker/dogs), say HR, LM(thieves and orcs) and LR (with say a bandit or an archer thrown in the mix). Which on paper (looking at the unit preferences) looks completely anti Blossoms, but I would fight that fight without any booster buildings (well I would feed the firebird) or second thoughts
You can find similar solutions for other units, like Frogs (the kings of the spire) and of course Rangers
In general 2 waves are only a problem if you have big losses in round 1. So if you expect big losses in wave one it is a good choice for convincing instead. If I see for example HR, M and multiple Mist Walkers I go right to the convince button. Fighting will always entail losses (or require silly amounts of booster buildings)