Dragon Quest Rivals Android/iOS/Switch - How to Play
Thanks to Jay on the forums for this writeup.
Each player starts with 25 HP. Reduce your opponent's HP to zero to win. You have six spaces to play your monster cards, divided into two rows of three. If monsters are lined up one behind the other, the one in the back row cannot be attacked. In the above example, Mandrake Major cannot reach Tuskateer because Grudgerigar is protecting it. Additionally, if you have three monsters across a whole column attacks cannot reach beyond them, including to your player character (I'll be refering to them as the leader from here on).
Monsters have 3 stats; The top left blue number is their cost. It will cost this much Mana to summon them. You start with one mana, and each turn the max increases by 1, and it fully replenishes. There seems to be no benefit to not using all your mana in a turn. The bottom left number in red is their attack stat, and the bottom right in green is their HP. When a monster's HP hits zero, they are destroyed and removed from the field. Monsters cannot attack on the turn they are summoned (with exception to ones that have the "Haste" abiliy.) When a monster attacks an enemy monster, the target will counter and both monsters will take damage. For example, if this Mandrake were to attack Terry's Crabid, Mandrake would take 3 points of damage. Slimes, Dragons, Zombies and named characters such as Maribel and Kyril will be denoted as such, as certain abilities specify them. Some monsters are exclusive to certain characters. The characters are: Terry, Jessica, Angelo, Alena, Torneko, Meena and Psaro.
Spell cards will activate immediatly, and simply have an Mana cost. This example is Buff and increases a monster's HP by 3. These are always exclusive to certain characters.The red and white colours signify Angelo.
Lastly, weapon cards. These equip directly onto the leader, and allow them to attack. Stats are just like a monster, Mana cost, power and HP. However HP for weapons simply decreases by one when the leader attacks, so it's more like uses. Also, unless the specific card says otherwise, a leader's attacks will still be countered if they attack a monster.
Once per turn, you can spend one Mana to Psyche Up, reaching three stacks of tension allows you to use your Coup De Grace, this is exclusive to each character. Angelo for example, uses Multiheal and restores 3 HP to himself and all ally monsters. This option is always available and doesn't require any special cards.
As you use a particular leader, they'll gain experience. Reaching certain thresholds will net you rewards such as gold or special cards exclusive to them.
Gold can be used to buy booster packs. Each pack contains six cards, and cards are usually exclusive to certain packs. (We have five up for purchase at this time.) You can also buy packs with tickets given as rewards, typically for daily log-ins, or with the premium currency. (It doesnt let me buy it, so I cant say how much a booster costs in real money).