Your question is pretty broad however, use them offensively to set up a full burst combo and confirm a kill, or use them to escape in a sticky situation. They pair well with cast time abilities, for example dark exchange on sorcerer.
Elswyer did introduce some new alchemy ingredients but I don't think any of them works with immovable. I still use the mag+health+immovable pots. Detect+mag+immovable if I come across a cloak spammer.