There are many strategies.
I used to play during the time that cards are rare and even normal gear is not so common back in 2005, xp was slow and dying was 3% - and we still leveled fine.
Firstly you need to choose your battles. Find monsters that are slow, low mdef. Then think of your strategy depending on your cast time / level / gear.
1. Firewall - Most efficient is a vertical firewall so monsters need to plow thru 3 pillars. If your damage is high enough, sometimes you dont need to cast another spell, just vertical firewall and they may die from it. Works great on alarms, stings, mi gao.
2. Safety wall - downside is you need blue gems, but nowadays its easy to make zeny. Use quagmire when you are a wizard to reduce monster aspd to prolong it. Can combine with some freezing strategy
3. Freeze - Many options here. 1. Sasquatch card. 2. Frost diver 3. Frost nova 4. Level 1 stormgust. Follow-up with a wind spell like JT or CL for efficiency. Downside is undead cant freeze.
4. Stone - Pest card. I wouldn't recommend stone curse or sienna execrate since its not very efficient (only was useful before for pvp, not so much anymore). Follow up with a fire spell.
5. Some gears and uninterruptable cast. - If you have neutral reduction gears like devilring, and your cast is not super long and you dont take too many hits, you can tank it till you cast. use a phen or geffen magic robe or something so the cast doesn't get interrupted. Then drain life to get your hp back.