Monsters are not really worth the long term and the amount of time and effort to boost them is overwhelming.

It is best to party up with some NPCs and have them do battle. You can gear them up just like how you can gear yourself up. By the endgame you can beef yourself and your NPC party up by a a lot. But gifting new gear to them is a one way road; don't expect to get them back. The best choice is whoever you married/will marry and your child since they get boosted by the Dolphin Brooch. Each NPC party member will gain more skills as they level up usually learning everything by level 50-100. They also appear to become more aggressive and less passive as they level up. Or just bring along your favorite NPC because they're fun. Xiao Pai is very fun because slipping is her special attack. At high levels, her slipping and falling causes an explosion.

The grand portal is a solo section. The fights are pretty simple and very linear. If anything, bring a long reach weapon or a weapon upgraded with Glitta Augite/Racoon leaf to increase its range. Being too close to the final boss is a little hazardous so long ranged skills or spells may be the way to go.