At least for DDoS by attacker who use Tor network, it can be mitigated by enabling PoW feature.
Only for onion service though. Forum would have to have a onion version and Tor users have to surf using that only.How demanding is it to make an onion version of the forum? Considering that these attacks last longer and longer, it would be really good to have an onion version that, if I understood correctly, would be resistant to such attacks.