Use betrayal-kill method. You can instant kill monsters if they have spared you. Make them spare, and then attack. You don't need to make it stay in the middle. (the slash vertical bar) Just click it wherever you like, but I recommend it to click it fast. If you have a high damage that makes the enemy die in just instant, you don't need to ACT them.

You need to buy pogs as much as you can buy. They are important, and they can increase chances of gold/diamond enemies spawn. Here is the tier:

Bronze: Requires 1 pog of it's own.

Silver: Requires 5 pog of it's own.

Gold: Requires 10 pogs.

Diamond: Requires 20 pogs.

Rainbow: Requires 99 pogs.

The maximum is 999 pogs.

Consider buy gold grinding rooms. They can also increasing chances of gold enemies spawn.

The list of the prices:

Ruins: 1000G

Snowdin: 1500G

Waterfall: 2000G

Hotland: 2000G

Dark World: 2000G

If you have got 45 Undercoins as freshly (received by fighting), then consider revive Bad tom. Fight him, then get 420G. If you just sell the Undercoins, then you only get 360G. Plus, you can't get the EXP.

If you already LV 20, buy the EXPx, as they would multiplies your EXP. You can stack it into X6. If you get Diamond Rudinn, and use X6, here is the EXP you got.

Diamond Rudinn: 300 XP.

300 X 6: 1800.

That means the maximum EXP you can get is 1800.

Since Rudinn have the largest EXP between the other monsters (except bosses, but I don't count because they don't have gold/diamond encounter).

That's it! Have fun!

Plus: Consider finish your bounty as they would give you money.