The Gardeneel Bros. are extremely weak both offensively and defensively, but they more than make up for it with a long-ranged attack that warps cats forward by 1375 range. While this ability does teleport your cats straight into their blind spot, it can also cripple your backline attackers and allow the enemies' front lines to advance with ease, making them a threat in any stage they appear in.
If there are not many enemy attackers, then you can try to snipe Gardeneel Bros. with any long-distance unit. There is not much trouble with him warping a stronger unit, as then that unit will kill Gardeneel Bros. but don't let your sniper get warped or it could be placed in somewhere with enemies in its blindspot.
Another way is to use high damaging meatshields like Maximum the Fighter or easily spammable attackers, as they could be warped into him and kill him off. This is a good strategy if he has a lot of support in the way. If he does not have support from the backline, you can try to get him to warp a high damaging unit such as Awakened Bahamut Cat to kill him and destroy the enemy base.