I don't see why that wouldn't work. I would normally advise against multiclassing for inexperienced players, but it would fit the idea of a potion making witch pretty well. I think it's more important that a character be fun and interesting than they be optimised, when bringing in new players. Three levels of alchemist and the rest warlock (fiend pact, obviously) would work well with this build. The
Enhanced Arcane Focus infusion +
Eldritch Blast should be awesome.
AS for playing a single class alchemist, bear in mind I have never played one, so I only know as much as
@Zardnaar. However, here are my thoughts:
Your role is
support. Do not expect to be in the thick of the action delivering smackdown. It's your job to provide the fighter with a magic sword, craft a
bag of holding when the party finds a big pile of loot, or
sending stones when they need to split up. Note that this is why
@Zardnaar thinks they are late blooming. If your campaign is the type where 2nd level characters can walk into a shop and come out with three magic weapons and a
bag of holding, then the artificer will be less useful at low level. However, if magic item distribution distribution is closer to a WotC adventure, then this is good at all levels. It's also worth noting that artificers excel in the exploration pillar. They can sub for a rogue at opening locks, and are often better than a rogue at dealing with traps. They are good for building a way across a chasm or a raging river of lava, and they are good at simply knowing stuff about practically anything. For this reason
Guidance is an important cantrip for them to know. It's also worth reading the descriptions of artisans tools.
Right Tool for the Job can be used to provide any part of these tool kits, which can then be abused for some different purpose entirely. For example Blacksmith's Tools contain a portable anvil, which can be used as a handy counterweight, or be dropped on an unsuspecting guard's head. Creativity is important for a good artificer player.
Homunculus Servant. Any artificer who doesn't get a pet from their subclass (Alchemist, Armorer) should get one of these ASAP. Not having one is like playing a chain pact warlock and not casting find familiar.
Elemental Elixir. Treat these like extra 1st level spells known. Also good for comedy.
Alchemical savant. Your
Healing Words are great for whack-a-mole. Note that High Elf is a good racial selection. If you give your character 16 dex and a magic rapier one thing you can do for fun is jump into battle with
Green Flame Blade. You have enough AC and hp to not be afraid of the front line, and you can use any of your infused items as a spellcasting focus.
Restorative Reagents. This is the alchemist's biggest weakness. You are really good at casting a spell that isn't needed very often. However, do note that
Lesser Restoration ends ghoul paralysis and Mind Blast stun. No need to wait for them to wear off.
I won't go on to high level stuff.