You're correct speed will only get you so far but usually there is at least 1 enemy team without a speed lead or a lesser version you can "gurantee" going 1st.  

Team 1 - Arbiter / Kymar / Deacon / Georgid

Team 2 - Mithrala / Pythion / UDK / Soulless

Team 3 - Cardinal / Ukko / Gnishaak / Magnaar

IMO those would be some decent teams with good synergy to start building towards.  The success of the go 2nd teams depends on your availability of your gear a bit though.  Stoneskin, Bolster, Shield, Reaction etc.

I'll add the biggest factor in progressing TTA is figuring out which team to use against enemy defenses.  Don't just blindly go in with the same order over and over, move your teams around so you give yourself the best match up against whatever the enemy is presenting.

This ^^^

Test and learn.  Half the teams you don't bother fighting because you are a chicken sh!t lil beoch 😀 you can beat right now. 

No sheep in silver, half probably don't set AI on defense, but most importantly you choose the matchups. And when you do lose, half of those you should be able to rematch in win based on already knowing the few hidden "advantages" the defense had on you (speed, stoneskin,...)

Defense, you need to look either fast, scary, and/or have rng (ss, ukko, reaction, revenge, ...).  The best defense is the one that doesn't get attacked, and can occasionally salvage a win with luck.

"The best defense is the one that doesn't get attacked, and can occasionally salvage a win with luck."

Soooo much this.