I suggest the simplest option: just ignore them and cast Darkness on more suitable targets (and if you have Sandro's Cloak, even better :smile10:), but if you really want to use Darkness specifically against elementals and phoenixes (by the way, if I'm not mistaken, Subdue works perfectly on phoenixes), then cast mass spells, especially Slow, which is effective due to the low initiative of elementals. Of course, there are different nuances, such as magic resistance, magic reflection, etc., but still, this is a kind of template.

P.S.: How are you going to win a hopeless battle with Summon when facing more or less substantial armies? In combination with other spells, it's possible, no doubt, but with just Summon alone?