When I first saw Axie infinity, the most confusing thing was positioning and targeting. Watching youtube videos, trying to understand why they use cards and why they stall. Energy and card attacks are easy to understand, but you can only really understand targeting after you play the game. Lets start with the default: two teams with 3 axies each, the closest to the middle is the Front liner or tank. The middle one is Midliner and the one next to the edge of the monitor is the Backliner. Same for the opponent, of course mirrored on his screen. There are 7 positions, I marked them with colors:
orange (looks really similar to red, sorry): it will attack randomly and it's first to get hit , so it's a risky position
yellow: it's less risky but still gets hit if orange is gone, top or bottom same but will attack the middle or the other yellow in front, then red then green in front then other red
red: it will attack top or bottom randomly if there is two
green, backline, will attack same as yellow if behind it, but is protected by yellow
Click the pictures to see more.
So there is not much to it, don't put your axies in middle, hug the top or bottom wall. Since your frontliner probably has lower damage cards, put him in middle, as it won't matter who it hits with his 1 or 2 offensive or semi-offensive cards. you can't put all of them on a side. The damage dealers on the back need to target the same axie so one behind the other. So that is only possible top or bottom, if you do middle they both attack randomly in case of 2 tanks. There are cards that interact with positioning, strawberry shortcake, silence whisper, mint. They are support cards and target the axie in front of them healing or cleansing him, but this also means their attacks will be random if you use them. Using 2 tanks only worth if both of them have high health and can heal back from damage, if the attacker falls for it, then he splits damage on both killing none, so it will be punished, especially by faster axies as they become even faster if they get hit. So that's why some people use 57 speed anemone tanks in 2 positions. 90% of the games the 3 axies are in a line.
The normal function of front liner is Tank, takes damage first so high shield cards, high health to save time for others. Healing, shield cards are best but some people use one or two attack cards too. Most people use energy cards, as when the 3v3 phase ends you might need more energy as you got enough cards. Since plants are highest health they are usually the tank, there are some aqua cards that are good for shield, stopping crits or healing back but also they are fast so some cards exploit hitting fast targets, like iguana or kotaro. Also they take 115% damage from plants so it's not a good omen putting them there unless you want them to die quick which is als oa strategy.
Midliners are usually there to protect the backline, gain energy, delay the game, until you can combo for the backliner. Midlaners get better card draw after the front dies, so they can use more advanced combos and get time to consistently pull it off. Any class can be a midliner, mostly bruiser style, as in 2v2 or 3v2 you get more cards repeating so high value cards are best. But also they can have the sole function of killing the opponents front line, like beasts. They will die in 3 hits after the front dies but they can combo kill the opponents tank by then. Ofc they work better with a sturdier tank with no damage but more health or healing or shields.
Backliners: usually most games go to 1v1, but at least 1v2 or 2v1, the backliner doesn't have that much risk of death so you can wait for specific combos until doing anything. So their job is usually to wait, and then they should go fast and should go hard for the kill, so most backliners are 4 attack cards or at least 3.
There are other strategies like sustain and survive until the blood moon starts taking away health from both axies. So there are cards that help healing, gaining energy, do more damage after later turns, usually if you can gain 1 energy or you got a 0 cost card, you can do 4 card combos as a backliner, otherwise you need to stock up energy before the 1v1 starts. And obviously going first in 1v1 has highest chance of win, especially if you got every card above 110-115-120 damage as most aquas got 420 health and maybe 100 shield. other backliners might be even less tanky but might be faster so there are multiple strategies and combinations, usually the mid and back should have different class cards to counter different opponents, or chain cards to buff each others attacks and shields.
Axie Infinity positioning has an interesting gameplay effect: you need to check speeds and calculate the optimal cards to kill a unit, each axie can target one single axie in a turn, if you kill the opponent axie the rest of cards won't be used to attack the one behind. So there are mind games of how much attack you need vs how much possible shield, you don't want overdoing it but you don't want to let it alive with a bit of health as you are wasting energy each time your cards not hit health. In defense your goal is raising enough shields to force your opponent using a card more, an energy more. if you can't do it, giving up your axie is the choice as he can't attack after he died. So killing fast axies is crucial, and giving up slow axies if you can't defend them for sure as these wasted cards can decide games. Also you might lose cards you didn't use, so it's best to have cards on front liners that are defensive and can be used every turn if needed, midliners can have situational cards or 0 cost ones to get strategical advantage.
This also means that most battles are front to back and frontlines got more health and shields while backliners can be more glass cannon. Read targeting for more details.