Unfortunately this seems to work only on Linux 64-bit
DISCLAIMER: It is always risky to flash your mobile device. I do not and will not take any responsibility for bootloops and bricks.
Please check carefully that the ROM or recovery.img you want to flash is suitable for your specific device and for the sp_flash_tool. The firmware directory should neither contain meta-inf nor system folder, but something_with_a_scatter.txt and a system.img
Backup your phone before flashing!
Your phone‘s battery should be at least half charged (50 percent) before you start this. 

I successfully ran these steps on my UMI Hammer with Linux Ubuntu 14.04. I do not have other devices or Linux derivates and do not plan to buy/install any.

CREDITS and a zillion thanks to Sergio Rivero for his excellent tutorial on mibqyyo.com!!!!
Also many thanks to N2K1 for giving me some more background information!

So here we go:

Step 1:
If you haven‘t done so before, install the Linux generic usb-drivers package:Download https://androidmtk.com/smart-phone-flash-tool
I recommend version 5.1604 or newer.
Step 2:
Extract the downloaded file -> graphic file manager (e.g. Nautilus): double-click and extract. Terminal:(from now on: please adapt paths to your own settings)
Move the new folder „sp_flash_tool“ to a directory of your choice.Step 3:

Change into the newly created directory:We make one file executable by typing:In order to avoid running the flash_tool as root user, you need to add a standard user to the usergroup "dialout" (thanks to @egalus for this bit!)
For Arch Linux:and activate the membership immediately
(Arch Linux):Step 4:

Now you can open the tool: 

or double-click on the flash_tool icon.
You can try at this stage if the flash tool connects to your phone:
In the user interface, choose tab „Download“. Hit "scatter-loading", navigate to a directory with a valid firmware for your device and choose the scatter-file.
For testing purposes uncheck "name" and check one of the smaller files in the list below (for example "logo") 
Switch off your device. Hit the "Download" button in SP_Flash_Tool and connect your phone to the computer. Some devices require you to take off the battery for about 10 seconds, with others you need to press Vol+ or Vol- while plugging the cable into the phone. Please look up device-specific threads and try out different options.

If nothing happens at all, open a second terminal, runand look out for a MediaTek entry. If there is none → did you install libusb-dev (Step 1?)
If the answer is yes, you might need to create a persistent udev rule for the MTK Preloader:Add the following line to the file:Save the file and exit.
Reload the usb-rules:Disconnect the usb data cable from the computer and the smartphone. Close the flash tool. Switch on your phone (fastboot mode will do). Repeat the first paragraph of Step 4.

If the tool connects, within a few seconds a red progress bar will appear. Unfortunately, in my case an error message came along with it:
S_BROM_CMD_JUMP_DA_FAIL (2035)
There is a connection, but also one more obstacle to overcome...

Step 5:
Now we come to the interesting part, which cost me 2 weeks of (futile) research and frustration. Finally I found Sergio Riveros tutorial on mibqyyo. Thanks to him again and again and again for this priceless piece of information 
To put it in different words: The modem manager controls port /dev/ttyACM0 and disables the Flash Tool. So we blacklist it for the two MTK vendor IDs the flash tool uses:You insert these two lines:Save the file, exit and restart udev:Switch your phone on (fastboot mode will suffice) and off again.

Step 6: 
or double-click on the flash_tool icon.
Now everything should run smoothly. In case you encounterchange the download agent to MTK_AllInOne_DA.bin (screenshot #4)
There are plenty of tutorials on the web for the further steps. 
Happy flashing! When a green circle with a white checkmark appears, you are done. You can close the flash tool and disconnect your phone. Remove the battery for about 10 seconds and reinsert it. For a clean start with a new ROM boot into recovery and wipe data/factory reset.