I'm not playing a no loss game, but I don't remember the frog being that hard. I think it hit me 2 times with the poison breath and killed 2 or 3 dragons total. Wasn't a big deal. As for the spider, I don't think it killed even one of my units, as I was just summoning demon portals like crazy. I had plenty of cannon fodder.
I'm noticing that my mage tactics are much different from the first game. Is it just me or has Demon Portal been nerfed, for example? I thought it used to summon way more units, especially with 70+ int. I also miss my two geysers a turn. Oh well.
Dragons seem stronger this time around, or maybe it's just and impression, as I haven't played the first one in a while. With shamans to heal my black & red dragons, I didn't lose any in the last 100 fights or so. They are truly incredible now.