Angel's skill is basically the same as Oswald's. When you activate it all the Tsums on the screen will change to two types: Angel and a 'Love-Struck' Stitch. One strategy to use after activating her skill is to quickly choose one type (hopefully the one with less Tsums if it is obvious which has less) and clear a few small chains quickly so that then you can create one big chain easily of the other type. It takes practice and obviously the longer time of higher skill levels is very helpful for this strategy.