The most useful monster.

No need! Berserkers aren't that fragile (18 hit points!) to require healing in the very first battle! Besides, you can position them further away and let them run around the hero and centaurs: they waste an extra turn and conserve their lives. Also, you always have to sacrifice some infantry in the early stages, no matter which faction you play.

It's not about them being fragile, but when playing as the Champion or Magister, a large part of them will fall in the very first battle; this is definitely true, but other factions don't have this problem. Some have archers providing cover, some have magic, and some have numbers on their side (for example, with Death, you can split the imps into squads, take a couple of small squads with you (1-2 each), and a large group of skeletons. By the end of the first week, you can gather over a hundred of them, and sometimes even more, depending on the map. Let the imps take the initial hit, and attack with the large group of skeletons; in this case, the losses are minimal, if not reversed). And some have both magic and archers. In short, everyone except the barbarians manages better here. Although, maybe I just don't know how to play as the barbarians