Angelic Sleipnir has a very fast and fairly powerful attack along with high health and range, but moves slowly. He has a small chance to knockback your cats, making him very deadly against poorly-prepared setups.
The best way to deal with this enemy is with Ramen Cat combined with cats that can outrange it, such as Ururun Cat, Dragon Cat, his Crazed counterpart or units with Long Distance. Rushers like A. Bahamut can also work well, but they are not reliable as often they have less range than Sleipnir. Do not bring Paris Cat, as she can't hit even once due to the shorter range and low health.