One thing to consider with the 1-slot giant beetle and fire beetle, is that you want to get their dex & stamina up to 125 before you raise it from 1 slot to 2. This will save you some training points. Dex & stamina will go up together to a max of 125 when raised through pet use, prior to training, but have to be raised separately once training that raises the pet's control slots.
So, you can train it on smaller stuff initially that give slow "training advancement" gains to raise skills, dex and stamina. The small orc forts in Malas are good for this. But, you'll only gain 0% to 0.2% per kill on those toward advancement.
Then, when the pet is at 125 dex/stamina, you can keep training skills, or start working on things that allow better advancement gains.
When the beetle is at 1 slot, and still has its original HP and resists, you don't want to have it taking the brunt of the hits from the thing it is training on, as it will die way too often. You need to get another pet that's 3-4 slots, or a friend with a 5-slot pet, and have it tank the target, with the beetle helping. You can gain about 4-8% per bird against a Swoop, 20-25% or more against the crazed mage in Shame, with another pet taking the brunt of the damage.
Once the beetle reaches 2 slots, and you raise the resists and HP (as well as HP regen), it might be able to take on the crazed mage with a LOT of vetting/healing. 3 slots or more, and it's likely to be able to be the tank for others, with far less healing, at which point you can use the crazed mages all the way to 5 slots if you like.