Use a MOSFET. Safely using an unregulated box mod without one will severely limit your power (wattage) options.
A series configuration can be safe if you know what you are doing. Running in series doubles your voltage, but keeps the same mAh and Amp limitations of an individual cell. You would have an operating voltage range of 8.4-7.0v (low end will depend on Amp load and batteries' safe discharge voltage).
A parallel configuration keeps the voltage of an individual cell, but doubles the mAh. In theory, it should double the Amp limit, but in practice it will be closer to a 40% increase. So in paralleling 30A batteries, you can expect a working limit of 42A.
You mention calculating for 150w, but don't say which method you planned on using. Amperage is based on resting voltage, wattage based on voltage drop under load. I only have graphs up to 20A, so I'll base this explanation on that. A fully charged battery reads ~4.2v. At 20A, the voltage under load is about 3.4v. So we'll do some math. You build a 0.21ohm coil and fire it on a fully charged battery. 4.2(resting voltage)/0.21(resistance)=20(Amps). 3.4(voltage under load)squared=11.56 11.56/.21(resistance)=55.05(watts).
Extrapolating:
The higher the Amperage, the greater the voltage drop under load. In a parallel configuration, you'll have a 42A working limit. 4.2(resting voltage)/42(Amps)=0.1(ohms). I don't have discharge graphs for 42A, but I'm going to assume the voltage under load to be around 3.0v. 3.0(voltage under load)squared=9 9/0.1(resistance)=90(watts).
In a series configuration, you'll have a 30A working limit. 8.4(resting voltage)/30(Amps)=0.28(ohms). I don't have discharge graphs for 30A in a series configuration. I usually experience around 1.2v drop at 17A, so I'll assume 1.8v drop at 30A. 6.6(voltage under load)squared=43.56 43.56/0.28(resistance)=155.57(watts).
In both cases of course, the wattage will drop as the resting voltage drops.
If this isn't how you calculated (and most people seem not to), then determine your current wattage output, and decide what your target wattage should be. Highest I've run was 144w on a regulated device. Even with straight VG it is a harsh vape. The people who tried it stopped after 4-6 hits. I regularly vape at 85-90w regulated, and can vape 144w fairly easily, but I really don't enjoy it much.
