I let Hector attack first and then counterattack for the kill. This way you will only receive damage when Hector attacks (once). Lucina alone should be able to tank them. If anything I can trap them where they can only attack Lucina, while Eirika and Nowi can buff Lucina from the sides. Lucina can take atleast 2 to 3 hits so that's already -2 or 3 Hectors. Then I swap her for Eirika once and she can take a hit or two. In that time Lucina should get the +10 heal and be able to tank yet another hit. So this way I should be able to take more than 4 Hectors out lol. Hell, even Nowi can tank a hit if necessary.