The third enemy is the medusa, literally treated as a boss encounter after the tutorial. Unless you think "This is serious, she's a purebred and super tough" does not elevate this monster. Also my healing isn't the issue with the second one, it's theirs. The second enemy you encounter in the game should not a drain skill that heals them to full while highly damaging you, that's stupid. "Not meant to kill all enemies" Yeah no shit, but you're still expected to kill some so you can, you know, level the fuck up.
weird glitch if she still petrified you, as said with the drainer though the heal of hers isn't really an issue
you should reach level 2 after clearing the three slimes and level 3 after like 3 drainers or 2 drainers 1 slime
the heal is based on the damage they deal which is about 70 or 2 lvl 1hits with her basic dealing less than the slime (so unless you were at 100 hp you wouldn't die in 2 turns and can just heal after getting hit by it, think she only uses it when she's low hp so shouldn't hit two in a row of those), not much of an issue unless you get very unlucky (for the few fights I did she rarely used it)
and as said the medusa is just a normal enemy that is used as the tutorial boss (the game tells you that it's a rare stray from another area), so the fact there are a lot in the jungle isn't unbalanced (you could defeat her at lvl 2 or 3 so defeating more when you are higher level isn't unbalanced)
there are healing skills you can get to not have to retreat to the bed (you also level up fast early on and I'd recommend the passive hp% regen skill) and there are better enemies for xp farm in the forest so you can just dodge all the medusas if you don't like them (I mostly skipped them because they were annoying when compared to the other enemies in the area that give more xp)
this is a jrpg so some grind is natural and compared to others I've played this is a pretty chill one (rebf gaiden from the same series is a lot harder than this one) so maybe I am just desensitized but nothing really felt unfair or overly annoying except like 1 or 2 bosses