Strategy to beat Snowflakes and Large Snowflakes

Honestly, I think everything everyone said last time more or less still applies. I'm not sure you will be able to do large snowflakes without sacrificing drops for better ce's (although it seems you're ce's are limited). Regular Snowflakes is mostly berserkers so you should be fine (although iffrit might be an issue).

As for xp, I'd save it, run the level, and decide based on that who to level. The most likely candidates are definitely your aoe lancers to take out Gilgamesh. I'm more partial to Larturia(she's a five star and fairly strong), but if you like Santa Lily go for it. Heracles is also great but you run the risk of him getting taken out by a (un)lucky crit. Large snowflake will be trickier due to the mix of classes.

You're really crippling yourself by spreading out the XP like that. One L70 archer is going to be a lot more useful than 3x L40 and 2x L30.

Long-term advice not related to this event: Max level one servant of every class. Then level servants so you have one single-target NP and one AOE NP. Level your tanks (Mash, Geogrios, Leonidas) so that you can rely on a support servant and keep them alive.

I think the only thing you can do is look for a friend support with a cu, cu alter, herc with bond ce, or scathach.

It's very satisfying to watch my NP3 Tama Lancer kill both Gils in one attack. Anti-male Lancer vs Archer == bad news. So, level your lancers. You can easily farm enough exp to max level them. There isn't no rush. Whether it takes you 5 turns or 25, just keep going and steadily upgrade your team's levels.