By the lack of useful monsters you should probably try to learn how to use "lower tier" monsters. There's a bunch of 2* and 3* monsters that are really important and those can help you getting through the dungeons with ease without being Nat5*.
Also it is more important to run the dungeon safely first. If you've done that you can try to get more speed in it (but that requires monsters that are a lot harder to get).
You should take some time farming a good team in order to tackle the dungeons properly. They're hard to beat for a reason. Also a few good monsters suitable for Giants can also be used elsewhere.
This is a good composition to tackle Giants B9 and 10, which should be your goal at the end of the day. For B8 you might want to bring something different. Veromos or Beretta as leader make things easier (instead of Konamiya). The other 4 mons are absolutely worth it in all three stages.
Monster | Where to get it
---- - -----
Bernard (Wind Griffon) | Tamor Desert, Mystical Scroll, Wind Scroll, ...
Konamiya (Water Garuda) | Unknown Scroll, Shop, Secret Dungeon,...
Darion (Light Vagabond) | Secret Dungeon
Belladeon (Light Inugami) | Secret Dungeon
Shannon (Wind Pixie) | Unknown Scroll, Shop, Secret Dungeon,...
As you can see you don't need any Nat4* or 5* to succeed in Giants.