Well Jayson the trick is to have 2 very good attack cards that can take out mamy of the commonly used cards. IE as you said Sniper Cecil makes a decent one. That gives you 28 cost. If you can manage a Pixie Archer or even better a gold Fianna make a strong 5 cost. Giving you 33 cost. You can get a 1 cost Pixie or even a Captain Eirny for 3 cost as a good card to sub out with your assist. If you've got strong assits like Linovah's you're pretty much set to win your battles. This puts you at about 34-36 cost leaving you for a decent 14-16 cost Be it a Humita or some other cost card for whatever reason.
There's plenty of times I'll sub in cards based on opponent to fill a specific role or counter a specific card.
This is just things I've learned as I've only been playing since mid Ruby Event. So many of the people I'm fighting tend to have at some times more than one silver/gold humita. Or level 60 SRs and such.
Basically newer players have a large uphill battle. Even if they just manage to get a Silver demon harpy it will give them 4 Mermaid kienna which they can max out for a substantial defense boost. Saw a guy with 5 gold ones on defense 33k+ defense.
I'll be honest with you mate, that Cecil's 4.7k attack at gold isn't going to cut it for some brawls. Even the newbs now have Gold Humitas, and Humita has 5k defense at gold. Although your idea is good, it's not perfect, since you're only going to get ~300 brawl points per battle, if you aim for a 3-2 win with a crew like that.
You'll really need some of the nice event cards maxed out (aka, Harpo for defense, Dark Ruby for attack) if you want to actually be competent in the ranks, because recently, a lot of high leveled players have been hopping around guilds in order to farm Humitas (I'm one of them, I'll admit, but I'm not as bad as this one player (of course, no naming), who's been hopping around to literally countless number of newbie guilds.
Finally, Mermaid crew isn't a very good idea. Mermaids have an absolute max of ~7k defense, which can rise slightly with combos, but that's it. Generally speaking, it's much better to put Nation-matching defense crews, because not only is the bonus larger, it's also more frequent. The guild-hopping player I mentioned above also has this setup, and it's actually not that hard to win against him; and he gives delicious amounts of brawl points per win.