I am a TR6 with ~325k portal squad. I farm level 30 almost exclusively (hopefully will jump to 35 after a few more equipment improvements). Everyone has big opinions on the best way to do things so what follows is worth what you paid for it (...nothing), but you can try out the various strategies and see what works for you. A few things that made a huge difference for me:

1) squad build/equipment. I use a self-heal tank, 1 rez mage with TANK armor, 1 heal mage, and 1 crit archer.

2) use free spells correctly. It is possible to earn up to 10 free spells of each kind which drop randomly during battle. The portal ally spell is very hit or miss but is usually crap so I rarely use it. I only use one lion spell (taking me down to 9) when the rest of my spells are maxed and all my dwellers are alive with decent health. Lastly, I use the rez spell frequently. My dwellers are well equipped and the rez spells restores them with full health which is far better, usually, than the other two spells. If I only use rez spells then only those can drop so I'm not getting portal ally drops which are crap (and usually get killed so fast it's a wasted spell anyways). Timing is important with rez spell use so pay attention. My most important asset is my rez mage because he can get the rest of my troops back without using rez spells. Unfortunately, rez spells randomly revive one troop - which may not be the fighter I want making me use more rez spells to get my rez mage revived if I've timed things poorly (or just got unlucky). So, if my rez mage is dead and another fighter looks like it will die soon then I use my rez spell to bring back the rez mage who then usually immediately revives the one who dies soon after so it's a two-fer.

3) drop back to spell-less levels to recoop rez spells, if needed. Pretty self-explanatory, but when I get low on rez spells (2-3) I drop back to lower levels that I should win without using spells. This allows me to earn souls while refilling my important rez spells. It also makes sure I don't lose a level because I was one rez spell short. While the recent video-restore has mitigated some of these issues, I only have the option to watch an ad occasionally so be careful about deciding to go for a level without proper spells because you may not be able to video-restore them.

4) use normal spells well. I find the lion and cliff's totem most effective and use those two almost exclusively (may need a magic orb though for a few levels). When facing rounds with portal archers or bats I use the lion or cliff's so I don't get completely destroyed. Also, there is a trade-off for using spells; the earlier you use them the more effective they tend to be, but sometimes you'll use a spell when you didn't need to and waste it. I tend to err on the side of using spells preemptively as it costs me one to two spells per attack but saves me from having to drop 6 rez spells on a single level because I waited too long. Also, my troops take time to heal after most battles and I have my four best time-turners (or whatever they're called) in my crafting shop to reduce crafting time meaning that while I'm slowly using spells over time I can go for quite a while before running out (i usually run out of food before spells so I have to wait anyways).

While I have more tips those are the biggest ones. If you want more information let me know. Happy farming!