1) Separate, isolated chargers. One for each cell. Yes, expensive.
2) Charge each cell separately, sequentially, using a single (STILL ISOLATED) charger. Yes, slow.
3) Dismantle the pack connections, verify similar cell voltages (within about 10-20mV) (important), then connect the cells in parallel, and charge as a one massive cell, using one charger.
4) Charge the pack as a series string (the normal way). If you have no BMS you can trust and there's a possibility the cells are not in balance (charge balanced at top), you act as a BMS by peeking around with a multimeter. Don't worry, they won't suddenly go bang in seconds. In a 16s pack, you can easily scan through the whole string in about one minute, which is quick enough. You can slow down or stop charging if any cell voltage starts rising considerably faster than the others or getting near the limit (say >3.5V for LFP). After the first cell hits the upper limit, go for method #2 for the rest of the cells. This is the fastest way and assuming you have little imbalance, you are done in an hour or two.
Diodes do not help you.