So to start this off, Yes I only purchased the Spellbook for $10 and I am working my way towards getting my ROI, and you can achieve it in a week or two especially if you like ccg's or collecting card games such as Hearthstone and Legends of Runeterra, and why not earn money while playing right? Oh and if you use my referral link, I will try and help you just let me know in the comments below.
The Story
Day 1 of purchasing the Spellbook, - I started off by gaining around 10 DEC, it was made possible by primarily using Marlic (Fire +1 Melee Attack) and Alric (Water +1 Magic attack) as my summoners. Note that different summoners will give different buffs to your monsters so feel free to test things out to find what works for you.
I recommend doing the Melee>Magic>Ranged comp unless the rules say otherwise. You can find out more about it here
Anyway, back to my main point, I lucked out in my daily chest, getting 100 DEC, which was more than enough to rent Mylor Crawling (Earth, Thorns - upon receiving damage from melee and ranged monsters, reflect some of the damage back, recommend that you rent him ASAP, it will help you earn more DEC), as well as the Javelin Thrower (Earth, 2 Mana 2, Attack, 5 Speed very good in the backline)
Day 2 - I earned 200 DEC that day, using mostly Mylor Crawling and Javelin Thrower - they tear up Bronze League, allowing you to accumulate win streaks left and right, and I got Sandworm (Melee, 5 Attack, 1 Speed, 5 Health) from my Daily Chest here, which is a very good common card.
Day 3-6 I mostly earned my DEC here mixing up Malric, Alric and Mylor, just mix them up and keep playing, remember to take note of what buffs they give so you can build your deck better, and remember to play around the rules too.
My Recommendations
Recommended Melees (should be placed in the first spot of the team, cannot attack otherwise unless specifically stated eg. Reach ability)
Fire - Cerberus - 4 Mana, 2 Attack, 3 Speed, Living Lava - 7 Mana, 3 Attack, 1 Speed, 2 Armor 6, Health
Nature - Unicorn Mustang - 8 Mana, 3 Attack, 4 Speed
Water - Frozen Soldier - 6 Mana, 2 Attack, 2 Speed, 5 Armor, 3 Health
Neutral - Elven Defender - 8 Mana, 2 Attack, 2 Speed, 3 Armor, 8 Health
Recommended Magic (generally in the middle of the team, magic attacks ignore armor)
Elven Mystic (neutral) - 4 Mana 1 Magic Attack 2 Speed
Recommended Ranged - Backline after melee and magic monsters
Javelin Thrower - Earth, 5 Mana 2, Attack, 5 Speed
Final Word
To end this, I would like to say that you might find something better that works for you, as my knowledge is only limited due to me still being a beginner but I can see potential in this game, plus its very fun, so try it out and I'll see you in game!
Cheers!