What Quix has to say about shard boosting...

I was asked also to expand on tips and tricks for low-level and unfinished map boosting techniques.

So to begin, it is possible to start shard boosting at level 25 and be quite successful, but it is not recommended, because this means you won't be able to play through all the levels of the game right away (or for quite awhile if you plan to do some serious low-level sharding).

It is easiest to have successful boosts as a monk, mainly because of the Haste skill, but early on, you won't have access to that skill (or it won't be as useful as the other skills that would serve you better). So how I set my skills at level 25 was the following:

Healbomb:5
Improved Healing:5
Wand Mastery:5 (can be substituted with weapon of choice)
Powerburst:5 (can be substituted)
Mass Heal:1
Bikram Yoga:4 (or whatever boosts your preferred stats)

Once you are strong enough to reach Grotto Isles, work your way up to "Wounded Caverns" and stop. Unlock Yetinothah Island by gathering all the necessary map scraps. On Yetinothah, work your way up to "Old Yeti Road" and stop. At level 25, these are the 2 that you would leave unfinished. This is because they spawn champs very frequently in the first room and you only have to peer into them to see if there is a warning or not. If not, move on to the other island and check that first room. If not, then you can start moving down the list of other zones on those maps that will yield easily accessible champs (i.e. don't spend much time searching these other zones):

Yeti:
Haunted Forest (just peek into the first room)
Bandage Man Road (just peek into the first room)
Flourishing Yeti Fields (check the first 2 rooms with enemies)

Grotto:
Whirlpool Caverns (first room only)
Sea of 100 Islets (first room only)
Valley of Spiders (first 2 rooms)
Shoreline (first 2-3 rooms)

Narwhalimbus (you rarely have to go back here):
Spiral Path (first 2 rooms)

You just keep going back to those 2 unfinished maps as they cooldown. Obviously, you'll be relying on summons to kill your champs for you, so make sure you have lots of high-level summons to lay waste to them. You won't have much HP, so kill enemies as you go until you reach the champ until you get more durable. This should yield about 300 shards per boost for those without a guild with perks.

Cash out shards for HP stones first. This is very important; you need to be able survive first, then work on damage stats.

By level 30, you'll have enough skill points to include Haste in your skill tree. Buy "Fake Fairie Wings" and obtain "Speed of the Fox" necklace from "Forest Clearing" on Sassafrass Island if you have not already. Move up to "Sasquish Road" (first 2 rooms only)on Yeti, and "Dredged Dunes" (first 3 rooms) on Grotto. Make sure you have some HP to withstand beatings from mobs at this level so you can run through faster. You should be able to run all other completed maps on G5 (highest level available) with the help of summons. Run through more rooms on the aforementioned Grotto Isles maps now that you have haste.

When you feel ready to move on, you can leave "Perilous Plains" on Yeti unfinished. Go to Mondomesa and be ready to get all the way up through AT LEAST the first 14 levels to "Bewildered Plains". This map will often yield champs in the first room, however there are not many other good maps to run prior to this map.

2 maps later, you can leave "Familiar Forest" unfinished and level up quickly using the 3 single coins found in the secret room there.

If you want to keep shard boosting, move up to "Makeout Point" and leave that unfinished. This gives you access to "Marshlands" and "Purged Field" which are 2 more decent maps to run. So at this point on Mondomesa, you can run:

Purged Field (first 2 rooms, more if you want but they're very long with low yield %)
Marshlands (first 4 rooms, same as above)
Bewildered Plains (first 3 rooms, and it's important to know your way around or be willing to give up champs in the 3rd room if they are not on the immediate island)
Tethered Rift (peek in)
Floating Isles (peek in)
Secret Garden (first 2 rooms)
...then move on back to Yetinothah.

Remember to always stop for 30 seconds from the moment you kill a champ until the moment you enter the next zone. You WILL NOT find champs within 30 seconds of a kill. This is a deliberate mechanism implemented by the developers.