Im in a top 100 which frequently goes top 50 and occasionally top 10.
Average war score is 245, dipped into sub-200 scores 3 times out of the last 30 wars.
I worked hard to get to that consistency and the major contributor to that is careful construction of my teams, and careful matching up to opponent teams. I have almost completely moved off mono (1 mono yellow per war left) with other teams ranging from 4-1 through to 2-1-1-1. As @Shunt says - the more tile options you have, the better. I am playing around with a fully rainbow reflect team at the moment that I will try to incorporate too. I have also made a conscious focus on working on my healers, with each team averaging 2 healers in different colors. Getting those healers (most of whom are 4*) maxed, emblemed and in some cases LB has really helped the overall effectiveness of my teams.
As mentioned I also try to look for synegies not just within my own team but also with the opponent teams. Dispels, cleanses, reflects, minion counters, buffers, battle of attirtion style heroes - all need to be factored in.
You might say that all of the above is common sense, but it is not just that - it took a long learning curve for me, lots of experimentation (raids are good for that) before I got the formula pretty right - and that is after 2 years+ of experience under my belt. And I am still learning, especially as new heroes are released so frequently.
Tile play also comes into it, but choosing the right team vs the right team gives you the best chances of turning poor-average boards around. And you have to play differently depending on which heroes you are facing and whether you are playing VF or another format. With many color options I tend to try to make moves from the top as much as possible in order to make multiple combinations wherever possible, charging simultaneously. I sometimes go for less appealing moves which give mana to more passive heroes (such as Freya or Bera) instead of a “better” move into a more dangerous hero like Frigg. With VF I play much more carefully, often avoiding diamonds and multi moves and just focusing on getting 3-tile matches.
I know theres a lot of high level stuff covered above, but it kind of has to start there…