Attaching terminals or splices to large gauge wire requires serious crimp tooling. By “large gauge”, I’m referring to 10AWG (5mm²) and larger wires. While you should always try to use the tooling recommended by the manufacturer of the terminal or splice, they can be prohibitively expensive for individual repairs or small runs.
Large wire generally requires more force to move the metal around than with smaller wires. Not only is the wire itself larger, but the terminals for larger wire tend to be made of thicker material to accommodate higher max current capabilities of the larger wire, too. Crimp tools with longer handles for leverage, ratcheting mechanisms, or other means of increasing mechanical advantage would tend to be more effective than tools without those features. Tools like the 2006KT-ND, WM5980-ND, and 288-1676-ND would be some examples.
When using crimp tools off label, I would recommend ordering extra terminals, and performing some test crimps to verify the process. Too little compression, and the wire will not make good contact with the terminal and could slip out of the crimp; too much compression, and the wire, terminal, or both can be damaged, causing a break at the crimp. In either case, the result would be a poor electrical joint, and likely a quick failure.