1. In general all the homuc class are worth it in terms of what you prefer. Considering you want a pet that can fend on its own Vanil or Filir (w/e that bird homuc class name is) would be your best option. Amistr is a pet that works like a tank in the long run.
2.Unlike Filir and Vanil at lvl 50, Amistr depending if its growth is at a good rating (randomly chosen when hatching) a lvl 50 Amistr still needs the guidance of its alchemist/bio partner such as going on flee build and having you tank it to fight monsters if you don't want to pot pitch every so often. Around lvl 65-75 is when its doing decent damage and survives a lot better.
3.Alchemist is a toughie class in the beginning considering you burn money on that class trying to keep up with potion usage and your own attack pot usage (if you use any at all examples: Acide and Fire demo pots). You can still try to do 1 on 1 fighting in toy factory lvl 2 alongside your pet or take a ice damascus weapon forged from blacksmith/whitesmith class to high orcs for awhile. Mostly I use fire demostration in niff with loli armor(auto heal) or majuroros/rybio (stun armor) and my geo healing, an just let the fire destroy the whole mob, while you have meat spam (having 2x Spring rabbit clips can help you if you use meat as healing).
4.Acid Demo works well in pvp, an mostly people just use it for pvp since it has a better use in that field (someone who pvps can well answer your question on that part). Fire Demostration is your god sent mobbing skill for undead races (mostly) an other monster races alike. Fire demo I use either in places like niff,prison,Glast heim, mostly undead/plant places to mob in. But it requires you to have meat/pots of any healing spammage to tank while the fire does its part. That's why armors like majuroros/rybio or any stun armor works well in this situation as you want to remain on your fire demo flame until the mob dies.
Heres a piccy of my fully grown Amistr (and its stats but its not the best ones) transformed if your still determined to train your Amistr:
(http://img4.imageshack.us/img4/8637/screenanthemro042n.jpg) (http://img4.imageshack.us/i/screenanthemro042n.jpg/)