Dunno if this is classed as abuse in the KoD, even though I have done it in the past (too lazy to now) but it is extremely easy to abuse it to maintain 1st place in the rankings, via having a friend/guild member fight and win/lose against said person and then tell the secondary person the cards said person has to beat them so
x,y - same guild
z - opponent
so x fights z (win/loses)
x tells y, z's card line up (stars and elements)
y swaps cards to element counter
y wins
And also another one is having multiple people lower there cards to the lowest they have allowing several people to attack from the same guild and win to then swapping there cards back to full strength, again:
a,b,c,d,e
a drops all cards to 3 star then b,c,d,e all fight and win then a swaps back to strong deck.
These both can be implied as strategy but at the same time it breaches the ToS.
6. Do not steal, scam, farm, or collect information from other player in the game. - example 1
9. Do not use multiple accounts to gain any advantages. - example 2 (seeing as multiple accounts doesn't specify if it has to be controlled by the same person)
NOTE: Yes I have admitted to doing both of these, but I kinda wanna see if it does come under breaking the ToS or not (curious)