Depends on the species. Typically you want to be using species types with access to magic if available. BLM is best but RDM and even DRK are still better than nothing. BLM+RDM is ideal.
You load up on instincts that buff magic damage (particularly earth if you have Stone II access) and run around oneshotting stuff with Stone II, then level up before you run out of MP. Scorp I, Sandworm III, Antlion II, Raffelesia II buff earth damage, but iirc the tradeoff isn't worth it for all of them. You are unlikely to run out of MP anyway between levels, but hecteyes III is good of you are having trouble with it. Flan II is generally great. Toad III helps with recast.
These III instincts aren't useful if you are trying to figure out how to level them from nothing. Just do the best you can with what you already have and work towards a better set fixing your current problems.
Some species lack types that have access to magic, but honestly not that many. In that case, you just load up on haste/MA and hit things until they die, which is typically one or two attack rounds. Remember to set the relevant RoEs.
The balance problem with monstrosity is multifold:
1. The extra 50 instinct capacity is the most valuable at level 30, where the marginal 50 points are spent on better instincts than they would be at 60 or 90, where you already have those good instincts equipped.
2. HP scaling means you can kill EPs at 30 way faster than EPs at 60 or 90
3. The elemental magic revamp really supercharged low tier spells because it was balanced to try and make lower tier spells more useful at 99.
This all pushes you towards grinding out monster levels at 30 in Bubu on a job with black magic.