As mentioned my m previous reply. Lowering the CD to 20 will make this card completely broken for Guild Wars due to potential of one-hit OK the boss.
I agree, I'm glad you are mentioning how overpowered cards can break the game. That's probably the biggest problem right now, Kirin hits for 6.6 million with Gretel and dk/dq leaders. Even without Gretel this card was hitting over 4 million on Azrael. This game was once prepanzer players and post panzer players, now its preKirin players and postKirin players.
A team consisting of a fully skilled 4* blue imp (easy to obtain imo), two echidna/naga or two 4* blue imps, Kirin, and Nerida with Hayyel helper will easily reach round 30 for Kirins activate, an then deal 2.4 million damage to 4 million damage easily defeating GW boss.
This team will reach round 30 and only take 14 attacks from GW boss, why should I continue to buy cards or strengthen my team, I now have a team that will never lose.
I scored over 420K points in last Nerida war as I enjoyed the more difficult GW boss as I felt better players were rewarded.
Making this game too easy is a real concern for me.