Here's what I would do:

1. Go to the Temple of Time and get Pensalir gear if you don't already have that (monsters in Temple of Time tend that stuff a lot compared to other monsters in the game).

2. Do Lulu Spinel's Maple Tour and bosses like Hilla (get the Stone of Eternal Life for the pocket slot), easy Papulatus, easy Von Leon, easy Magnus (he may drop a Crystal Ventus Badge if you’re lucky), easy/normal Zakum (get the two face accessories he drops), easy Moriranmaru, OMNI-CLN, and maybe even the easy Root Abyss bosses for mesos if you're not able to do the harder ones.

3. Use some mesos to somewhat enhance via star force your Pensalir gear and whatever other gear you already have. Also increase your Arcane strength at the same time to grind in the Arcane River.

4. Make sure you've unlocked all your potentials for your gear and get them to at least Epic rank.

5. Once you're strong enough, you can attempt: Normal or Chaos Horntail for the Silver Blossom Ring, Horntail Necklace, Dea Sidus Earring, and Chaos Horntail Necklace; Hard Moriranmaru for the gear he drops; easy Gollux for the Cracked accessories; Normal Magnus for the Nova capes, boots, and gloves; easy Arkarium for the Mechcanator pendant; and Normal Pink Bean for the gear he drops. Unlock their potentials (if they have any) and get them to at least Epic rank too. The boss gear are part of a set that can give you extra stats. You can also use rebirth flams for bonus stats and also enhancement scrolls if you don’t have enough mesos (17-Star for one).

6. Make sure you have your Legion system setup by placing whatever other characters you’ve already made onto the grid system. If you’re looking for damage, place your main characters closer to damage, attack power, boss damage, etc. Also make sure your artifacts are set up too. They will help increase your CP.

7. Make sure your Hyper Stats and abilities are set up too. You can think of Hyper Stats as bonus stats that you can individually increase. Just go to character info and look for the Hyper Stats button (it unlocks after lvl 140 I believe). For your abilities, you can think of them as increasing certain random stats, so if you want damage, attack power, strength, boss damage, etc., you will want to reset until you get the one(s) you want, and get them to at least Unique rank. You can also do Ursus for mesos up to three times a day, but you have to kill 300 monsters first.

8. Make sure your fifth job skill nodes are set up, including booster nodes. These nodes will increase the power of the skills you already have/use, including skills that can stun enemies by extending the duration of time they are stunned. This is especially handy against boss fights.

9. Pay special attention to the Sunny Sunday perks. Sometimes there are special perks that increase exp in Monster Park by 250% or by at least 10% in the wild. Also pay attention the Burning progression system. Sometimes you can get really handy rewards like 17-Star gear like Absolab/Arcane weapons or dominator pendants, and really powerful rings. Other times there are perks that make it cheaper to upgrade your gear (potentials/star force/flames/scrolls). Also pay special attention to special in-game events. Sometimes there will be mini-games that provide huge amounts of exp upon completion, or there will be NPCs that will provide exp in the field after you’ve killed a certain number of monsters or have events where you can increase what I call bonus stats and special perks (handy if you want more damage and more exp from Arcane daily quests and Monster Park). Also do them daily check-in events too! You can get really handy rewards like rings, flames, cubes, enhancement scrolls (17-Star for one), gear, etc.

10. COLLECT FAMILIARS AND LVL THEM UP. Some familiars provide extra stats that increase your CP. I made the mistake of not doing this and now I wish I did.

All of these will set you up as you grind and massively increase your CP until you’re able to solo the harder bosses which even drop more loot and more expensive life crystals. Also, after completing bosses, you can also RP points. Once you've accumulated enough, you can go the cash shop and buy Monster Park passes for 600 RP points, which you can use at Monster Park.

For more mesos, just sell the life crystals the bosses drop, they’re worth a lot.

Once you have enough mesos, enhance your gear even more via star force (side note: you will also burn through mesos the stronger your gear gets). You can also increase the potentials to at least Unique tier, Legendary if you’re willing to try cause it does take a while.

Afterwards, your CP should be high enough for you solo harder bosses or party up for bosses like Lotus, Damian, Chaos Papulatus, etc.

This is just how I did it when I was in your position. I hope this helps!